[llvm] [InstCombine] Fold `fcmp (fmul X, C1), C2` to `fcmp X, C2/C1` (PR #205003)

[Matt Arsenault via llvm-commits]

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arsenm wrote:

The denormal check doesn't feel right, the problem seems to be a more general hazard around underflow. [This](https://alive2.llvm.org/ce/z/KxiXc_) fails to verify with IEEE since the result is still 0 

https://github.com/llvm/llvm-project/pull/205003