[llvm] [InstCombine] Fold more 'fcmp' 'select' instrs idioms into 'fabs' (PR #83381)

[Joshua Cranmer via llvm-commits]

jcranmer-intel wrote:

(Sorry for the delay, I decided it was more prudent to get my FMF RFC out before making any further comments on this PR).

Overall, I don't think this is the right place to do the checking on function-level attributes. Instead, we should probably be having SROA propagate function-level attributes to appropriate FMF (the actual PHI node creation happens in lib/Transforms/Utils/PromoteMemoryToRegister.cpp, not SROA itself), rather than doing one particular query in one particular pattern in InstCombine.

https://github.com/llvm/llvm-project/pull/83381