AMDGPU: Add a few missing mfma rewrite tests

Test other splitting situations that appear in greedy.
This includes ensuring we have a case that hits a local split
and instruction split (most of the tests hit the region split path).

Also test a few cases where the final result isn't fully used, resulting
in partial copy bundles instead of a simple full copy. Test physreg
and virtreg agpr interference with a reassignment candidate.

I'm accumulating too many failure cases, and MIR tests are very prone
to painful merge conflicts, so I've added a few more tests and extracted
new tests from #147975.

Closes #149026

Author: arsenm