AMDGPU: Partially merge indirect register write handling

a785209b switched to using a pseudos instead of manually tying
operands on the regular instruction. The VGPR indexing mode path
should have the same problems that change attempted to avoid, so these
should use the same strategy.

Use a single pseudo for the VGPR indexing mode and movreld paths, and
expand it based on the subtarget later. These have essentially the
same constraints, reading the index from m0.

Switch from using an offset to the subregister index directly, instead
of computing an offset and re-adding it back. Also add missing pseudos
for existing register class sizes.