If we ended up with two phi instructions in a block, and we needed to fix up the banks for the first one, we'd end up inserting our COPY before the second phi.
%x = G_PHI ... %fixup = COPY ... %y = G_PHI ...
This is invalid MIR, and breaks assumptions made by the register allocator later down the line. With the verifier enabled, it also emits a verification error.
This teaches fixupPHIOpBanks to walk past any phi instructions in the block when emitting the fixup copies.
Here's an example of the crashing code (same as added testcase):