[AMDGPU] Remove dead frame indices after sgpr spill.

All those frame indices which are dead after sgpr spill should be removed from
the function frame. Othewise, there is a side effect such as re-mapping of free
frame index ids by the later pass(es) like "stack slot coloring" which in turn
could mess-up with the book keeping of "frame index to VGPR lane".

Reviewed By: cdevadas

Differential Revision: https://reviews.llvm.org/D111150