1ceccbb0dd
If an undef subregister def is live into another block, we need to maintain a physreg def to track the liveness of those lanes. This would manifest a verifier error after branch folding, when the cloned tail block use no longer had a def. We need to detect interference with other assigned intervals to avoid clobbering the undef lanes defined in other intervals, since the undef def didn't count as interference. This is pretty ugly and adds a new dependency on LiveRegMatrix, keeping it live for one more pass. It also adds a lot of implicit operand spam (we really should have a better representation for this). There is a missing verifier check for this situation. Added an xfailed test that demonstrates this. We may also be able to revert the changes in 47d3cbcf842a036c20c3f1c74255cdfc213f41c2. It might be better to insert an IMPLICIT_DEF before the instruction rather than using the implicit-def operand. Fixes #98474
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