AMDGPU: Fix treating unknown mem operands as uniform (#170309)
The test changes are mostly GlobalISel specific regressions.
GlobalISel is still relying on isUniformMMO, but it doesn't really
have an excuse for doing so. These should be avoidable with new
regbankselect.
There is an additional regression for addrspacecast for cov4. We
probably ought to be using a separate PseudoSourceValue for the
access of the queue pointer.