AMDGPU: Use removeAllRegUnitsForPhysReg()

I met the issue here when working on something else.
Actually we have already reserved EXEC, but it looks
like the register coalescer is causing the sub-register
of EXEC appears in LiveIntervals. I have not looked
deeper why register coalscer have such behavior, but
removeAllRegUnitsForPhysReg() is the right way.

Reviewed By: critson, foad, arsenm

Differential Revision: https://reviews.llvm.org/D117014