Re-commit r269828 "X86: Avoid using _chkstk when lowering WIN_ALLOCA instructions"

with an additional fix to make RegAllocFast ignore undef physreg uses. It would
previously get confused about the "push %eax" instruction's use of eax. That
method for adjusting the stack pointer is used in X86FrameLowering::emitSPUpdate
as well, but since that runs after register-allocation, we didn't run into the
RegAllocFast issue before.

llvm-svn: 269949