[PATCH] Win64 ABI shouldn't extend integer type arguments.

[Julien Lerouge]

One more update to unbreak CodeGenCXX/microsoft-abi-member-pointers.cpp in r216507. We can preserve the zeroext on bool types, that still gives us code compatible with MSVC, since it's only extending to 8bits.

http://reviews.llvm.org/D4380