[llvm] [KeyInstr][DwarfDebug] Add is_stmt emission support (PR #133495)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams commented:

> I get the impression that GroupCandidates and KeyInstructions are being kept strictly in sync; thus couldn't one instead just load KeyInstructions from GroupCandidates once the scan is complete? This avoids filling up the dense map with tombstones.

Good observation. That has enabled some code simplification too.

> Am I right in understanding that the buoy means the "least precedence" instruction will get the is_stmt if the highest precedence appears after it in the contiguous blob? (Seems fine, just making sure I understand).

Yep that's right.

> On the whole, the computation function feels like it could be simpler, but in some intangible way I'm not immediately sure of. (Still reading the tests).

This part of the code changed and grew a few times during prototyping; despite efforts to keep it away from the clean/final version some of that cruft evidentially made it into this patch. Looking at it with fresh eyes I've simplified it a bunch (I've split the clean up into few commits so it's easy to track how things moved about).

https://github.com/llvm/llvm-project/pull/133495