Recommitting r275284: add support to inline __builtin_mempcpy

Patch by Sunita Marathe

Third try, now following fixes to MSan to handle mempcy in such a way that this commit won't break the MSan buildbots. (Thanks, Evegenii!)

llvm-svn: 277189
diff --git a/llvm/lib/Transforms/Utils/BuildLibCalls.cpp b/llvm/lib/Transforms/Utils/BuildLibCalls.cpp
index f4260a9..e61b04f 100644
--- a/llvm/lib/Transforms/Utils/BuildLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/BuildLibCalls.cpp
@@ -250,6 +250,7 @@
     Changed |= setDoesNotCapture(F, 2);
     return Changed;
   case LibFunc::memcpy:
+  case LibFunc::mempcpy:
   case LibFunc::memccpy:
   case LibFunc::memmove:
     Changed |= setDoesNotThrow(F);