[AMDGPU] Add pass to rewrite partially used virtual superregisters after...

[AMDGPU] Add pass to rewrite partially used virtual superregisters after RenameIndependentSubregs pass with registers of minimal size.

The main purpose of this is to simplify register pressure tracking as after the pass there is no need
to track subreg liveness anymore.

On the other hand this pass creates more possibilites for the subreg unaware code, as many of the subregs
becomes ordinary registers.

Intersting sideeffect: spill-vgpr.ll has lost a lot of spills.

Reviewed By: #amdgpu, arsenm

Differential Revision: https://reviews.llvm.org/D139732