Optimize the mbs fast path slightly.

From a logcat profile:
```
     |--95.06%-- convertPrintable(char*, char const*, unsigned long)
     |    |--13.95%-- [hit in function]
     |    |
     |    |--35.96%-- mbrtoc32
     |    |    |--82.72%-- [hit in function]
     |    |    |
     |    |    |--11.07%-- mbsinit
     |    |    |
     |    |    |--5.96%-- @plt
```
I think we'd assumed that mbsinit() would be inlined, but since these
functions aren't all in wchar.cpp it wasn't being. This change moves the
implementation into a (more clearly named) inline function so we can
trivially reclaim that 11%+6%.

Benchmarks before:
```
-------------------------------------------------------------------
Benchmark                         Time             CPU   Iterations
-------------------------------------------------------------------
BM_stdlib_mbrtowc_1            8.03 ns         7.95 ns     87144997
BM_stdlib_mbrtowc_2            22.0 ns         21.8 ns     32002437
BM_stdlib_mbrtowc_3            30.0 ns         29.7 ns     23517699
BM_stdlib_mbrtowc_4            37.4 ns         37.1 ns     18895204
BM_stdlib_mbstowcs_ascii     792373 ns       782484 ns          890 bytes_per_second=609.389M/s
BM_stdlib_mbstowcs_wide    15836785 ns     15678316 ns           44 bytes_per_second=30.4138M/s
```

Benchmarks after:
```
-------------------------------------------------------------------
Benchmark                         Time             CPU   Iterations
-------------------------------------------------------------------
BM_stdlib_mbrtowc_1            5.76 ns         5.72 ns    121863813
BM_stdlib_mbrtowc_2            17.1 ns         16.9 ns     41487260
BM_stdlib_mbrtowc_3            24.2 ns         24.0 ns     29141629
BM_stdlib_mbrtowc_4            30.3 ns         30.1 ns     23229291
BM_stdlib_mbstowcs_ascii     783506 ns       775389 ns          903 bytes_per_second=614.965M/s
BM_stdlib_mbstowcs_wide    12787003 ns     12672642 ns           55 bytes_per_second=37.6273M/s
```

Bug: http://b/206523398
Test: treehugger
Change-Id: If8c6c39880096ddd2cbd323c68dca82e9849ace6

libc/bionic/c16rtomb.cpp

@@ -43,7 +43,7 @@ static inline constexpr bool is_low_surrogate(char16_t c16) {
 size_t c16rtomb(char* s, char16_t c16, mbstate_t* ps) {
   static mbstate_t __private_state;
   mbstate_t* state = (ps == nullptr) ? &__private_state : ps;
-  if (mbsinit(state)) {
+  if (mbstate_is_initial(state)) {
     if (is_high_surrogate(c16)) {
       char32_t c32 = (c16 & ~0xd800) << 10;
       mbstate_set_byte(state, 3, (c32 & 0xff0000) >> 16);

libc/bionic/c32rtomb.cpp

@@ -50,7 +50,7 @@ size_t c32rtomb(char* s, char32_t c32, mbstate_t* ps) {
     return mbstate_reset_and_return(1, state);
   }
 
-  if (!mbsinit(state)) {
+  if (!mbstate_is_initial(state)) {
     return mbstate_reset_and_return_illegal(EILSEQ, state);
   }
 

libc/bionic/mbrtoc32.cpp

@@ -55,7 +55,7 @@ size_t mbrtoc32(char32_t* pc32, const char* s, size_t n, mbstate_t* ps) {
   }
 
   uint8_t ch;
-  if (mbsinit(state) && (((ch = static_cast<uint8_t>(*s)) & ~0x7f) == 0)) {
+  if (mbstate_is_initial(state) && (((ch = static_cast<uint8_t>(*s)) & ~0x7f) == 0)) {
     // Fast path for plain ASCII characters.
     if (pc32 != nullptr) {
       *pc32 = ch;
@@ -105,7 +105,7 @@ size_t mbrtoc32(char32_t* pc32, const char* s, size_t n, mbstate_t* ps) {
   size_t bytes_wanted = length - bytes_so_far;
   size_t i;
   for (i = 0; i < MIN(bytes_wanted, n); i++) {
-    if (!mbsinit(state) && ((*s & 0xc0) != 0x80)) {
+    if (!mbstate_is_initial(state) && ((*s & 0xc0) != 0x80)) {
       // Malformed input; bad characters in the middle of a character.
       return mbstate_reset_and_return_illegal(EILSEQ, state);
     }

libc/bionic/wchar.cpp

@@ -54,7 +54,7 @@
 //
 
 int mbsinit(const mbstate_t* ps) {
-  return (ps == nullptr || (*(reinterpret_cast<const uint32_t*>(ps->__seq)) == 0));
+  return ps == nullptr || mbstate_is_initial(ps);
 }
 
 size_t mbrtowc(wchar_t* pwc, const char* s, size_t n, mbstate_t* ps) {
@@ -148,7 +148,7 @@ size_t wcsnrtombs(char* dst, const wchar_t** src, size_t nwc, size_t len, mbstat
   static mbstate_t __private_state;
   mbstate_t* state = (ps == nullptr) ? &__private_state : ps;
 
-  if (!mbsinit(state)) {
+  if (!mbstate_is_initial(state)) {
     return mbstate_reset_and_return_illegal(EILSEQ, state);
   }
 

libc/private/bionic_mbstate.h

@@ -44,6 +44,10 @@ __BEGIN_DECLS
 #define __MB_IS_ERR(rv) (rv == __MB_ERR_ILLEGAL_SEQUENCE || \
                          rv == __MB_ERR_INCOMPLETE_SEQUENCE)
 
+static inline __wur bool mbstate_is_initial(const mbstate_t* ps) {
+  return *(reinterpret_cast<const uint32_t*>(ps->__seq)) == 0;
+}
+
 static inline __wur size_t mbstate_bytes_so_far(const mbstate_t* ps) {
   return
       (ps->__seq[2] != 0) ? 3 :