Factor out redzone ABI checks [NFCI]

As requested in D58632, cleanup our red zone detection logic in the X86 backend. The existing X86MachineFunctionInfo flag is used to track whether we *use* the redzone (via a particularly optimization?), but there's no common way to check whether the function *has* a red zone.

I'd appreciate careful review of the uses being updated. I think they are NFC, but a careful eye from someone else would be appreciated.

Differential Revision: https://reviews.llvm.org/D61799

llvm-svn: 360479