Target/X86: Fix assertion failures and warnings caused by r151382 _ftol2...

Target/X86: Fix assertion failures and warnings caused by r151382 _ftol2 lowering for i386-*-win32 targets. Patch by Joe Groff.

[Joe Groff] Hi everyone. My previous patch applied as r151382 had a few problems:
Clang raised a warning, and X86 LowerOperation would assert out for
fptoui f64 to i32 because it improperly lowered to an illegal
BUILD_PAIR. Here's a patch that addresses these issues. Let me know if
any other changes are necessary. Thanks.

llvm-svn: 151432