[msan] Don't check divisor shadow in fdiv.

[eugenis]

Summary

Floating point division by zero or even undef does not have undefined
behavior and may occur due to optimizations.

Fixes https://bugs.llvm.org/show_bug.cgi?id=37523.

[kcc]

LGTM

lib/msan/tests/msan_test.cc

 typedef U4 V4x32 __attribute__((__vector_size__(16)));
 typedef U8 V2x64 __attribute__((__vector_size__(16)));
 typedef U4 V8x32 __attribute__((__vector_size__(32)));
 typedef U8 V4x64 __attribute__((__vector_size__(32)));
 typedef U4 V2x32 __attribute__((__vector_size__(8)));
 typedef U2 V4x16 __attribute__((__vector_size__(8)));
 typedef U1 V8x8 __attribute__((__vector_size__(8)));
 
-
 V8x16 shift_sse2_left_scalar(V8x16 x, U4 y) {
   return _mm_slli_epi16(x, y);
 }
 
 V8x16 shift_sse2_left(V8x16 x, V8x16 y) {
   return _mm_sll_epi16(x, y);
 }
 

test/msan/vector_div.cc

+// Regression test for https://bugs.llvm.org/show_bug.cgi?id=37523
+
+// RUN: %clangxx_msan -O0 %s -o %t && %run %t
+// RUN: %clangxx_msan -O3 %s -o %t && %run %t
+// REQUIRES: x86_64-target-arch
+
+#include <assert.h>
+#include <emmintrin.h>
+
+int main() {
+  volatile int scale = 5;
+  volatile auto zz = _mm_div_ps(_mm_set1_ps(255), _mm_set1_ps(scale));
+  assert(zz[0] == 51);
+  assert(zz[1] == 51);
+  assert(zz[2] == 51);
+  assert(zz[3] == 51);
+}