[AMDGPU] Fix register class for a subreg in GCNRewritePartialRegUses.

1. Improved code that deduces register class from instruction definitions. Previously if some instruction didn't contain a reg class for an operand it was considered as no information on register class even if other instructions specified the class.

2. Added check on required size of resulting register because in some cases classes with smaller registers had been selected (for example VReg_1).

Reviewed By: arsenm, #amdgpu

Differential Revision: https://reviews.llvm.org/D152832