AMDGPU: Add test for m0 initialization in basic loop

Initialization of m0 is emitted for each LDS operation, so
every block with LDS usage ends up with one. MachineLICM
used to fail to hoist this out of the loop, so every loop
iteration with LDS usage in it would re-initialize it.

This seems to be fixed now, so add a test to make sure that
it stays this way.

llvm-svn: 266156