[AMDGPU] Run SIShrinkInstructions before post-RA scheduling

Run post-RA SIShrinkInstructions just before post-RA scheduling, instead
of afterwards. After the fixes in D112305 and D112317 this seems to make
no difference, but it paves the way for scheduler tweaks that are
sensitive to the e32 vs e64 encoding of VALU instructions.

Differential Revision: https://reviews.llvm.org/D112341