X86RegisterInfo: eliminateFrameIndex: Force SP for AfterFPPop; NFC

AfterFPPop is used for tailcall/tailjump instructions. We shouldn't ever
have frame-pointer/base-pointer relative addressing for those. After all
the frame/base pointer should already be restored to their previous
values at the return.

Make this fact explicit in preparation for an upcoming refactoring.

Differential Revision: https://reviews.llvm.org/D32205

llvm-svn: 300922