[llvm] Use DIExpression::foldConstantMath() at the result of an append() (PR #71719)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse approved this pull request.

LGTM with nits. I imagine it's slightly inefficient to DIExpression::get and unique one expression, then do it again on another, it might be worth pushing this to the compile-time-tracker to have a look first.

https://github.com/llvm/llvm-project/pull/71719