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

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

There is a failure with this pass in the case when target register class for a subregister isn't known from instruction description (for ex. COPY).
Currently in this situation the RC is obtained using TargetRegisterInfo::getSubRegisterClass but in general it's not working.

In order to fix this two things should be done:
1. Stop processing a subregister if the target register class is unknown (conservative approach)
2. Improve deduction of subregister' target register class (i.e by processing COPY chain)

I was going to implement point 1 but my tests use implicit operands for S_NOP and they don't have associated target register class and all tests fail.
Therefore I decided to turn off the pass now, implement point 1 and fix my tests.

Reviewed By: arsenm, #amdgpu

Differential Revision: https://reviews.llvm.org/D152291