[AMDGPU] Add basic patterns to select lshl_or instead of v_perm

[jrbyrnes]

It does the operation using fewer sreg.

Also, enables some future commits.

[arsenm]

Can we avoid emitting a perm in the first place? Can we decompose existing perms into the shift + or? Does computeKnownBits know about AMDGPUperm?

[jrbyrnes]

It was an attempt to delete the hasNon16BitAccesses heuristic, but, after further thought, the heuristic is the correct approach.

This approach is not even correct as it stands: the upper 16 bits of the non-shifted source need to either be provably 0 or masked -- in the latter case, it is better to use a perm. It seems the better approach is to capture the last statement in the heuristic, and avoid / emit the perm accordingly.

computeKnownBits does know about AMDGPUPerm, but there is an outstanding issue.