[llvm] [DAGCombiner] Reassociate FMA chains sharing an FMUL (PR #217496)

[Justin Fargnoli via llvm-commits]

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justinfargnoli wrote:

Doesn't the `fmul` need `contract`/`isContractableFMUL()` and the `fma` need `reassoc`? 

CC @LewisCrawford @jcranmer-intel since I'm not super familiar with floating-point things. 


https://github.com/llvm/llvm-project/pull/217496