[llvm] [DAGCombiner][GlobalISel] NFC: Extract isFusedOp, add FMA contraction test, fix isFPExtFoldable (PR #189093)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Fix missing isFPExtFoldable check in GISel

A bug fix like this should really go in a separate PR with tests that clearly show the change from old (incorrect) to new codegen.

https://github.com/llvm/llvm-project/pull/189093