[AMDGPU][GFX11] Legalize and select partial NSA MIMG instructions

If more registers are needed for VAddr then the NSA format allows then the
final register can act as a contigous set of remaining addresses. Update
legalizer to pack register for this new format and allow instruction
selection to use NSA encoding when number of addresses exceeds max size.
Also update SIShrinkInstructions to handle partial NSA.

Differential Revision: https://reviews.llvm.org/D144034