[X86] Remove side-effects from determineCalleeSaves

(Target)FrameLowering::determineCalleeSaves can be called multiple
times. I don't think it should have side-effects as creating stack
objects and setting global MachineFunctionInfo state as it is doing
today (in other back-ends as well).

This moves the creation of stack objects from determineCalleeSaves to
assignCalleeSavedSpillSlots.

Differential Revision: https://reviews.llvm.org/D41703

llvm-svn: 321987