This was checking for default operands in the current DAG instruction,
rather than the correct result operand list. I'm not entirly sure how
this managed to work before, but was failing for me when multiple
default operands were overridden.
Copy the logic from the existing handling in the DAG matcher emittter.
This will enable some AMDGPU pattern cleanups without breaking
GlobalISel tests, and eventually handle importing more patterns.
The test is a bit annoying since the sections seem to randomly sort
themselves if anything else is added in the future.