[llvm] Fix assert in SlotIndexes::getInstructionIndex in debug builds (PR #189370)

[Carl Ritson via llvm-commits]

https://github.com/perlfu commented:

Needs test.

High-level presumably the issue occurs because `::toExact` is passed a debug instruction in `Before`?


https://github.com/llvm/llvm-project/pull/189370