[X86] Don't clobber reserved registers with stack adjustments

Summary:
Calls using invoke in funclet based functions are assumed to clobber
all registers, which causes the stack adjustment using pops to consider
all registers not defined by the call to be undefined, which can
unfortunately include the base pointer, if one is needed.

To prevent this (and possibly other hazards), skip reserved registers
when looking for candidate registers.

This fixes issue #45034 in the Rust compiler.

Reviewers: mkuper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39636

llvm-svn: 317551