[AMDGPU] Add v5f32/VReg_160 support for MIMG instructions

Avoid having to round up to v8f32/VReg_256 when only 5 VGPRs are
required for a MIMG address operand.

Maintain _V8 instruction variants of pseudo instructions allowing
assembly prior to GFX10 to work as-is.  Currently the validator
can tell for GFX10 what the correct size is, so will disallow
oversize address registers.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D103672