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

[Lewis Crawford via llvm-commits]

================

----------------
LewisCrawford wrote:

Correction:

- FMul needs `contract`
- FMA needs `reassoc`
- FAdd needs `reassoc` and `contract`
So Justin's suggestion is right.

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