[llvm] [SelectionDAG] Give the contract PatFrags a GlobalISel predicate (PR #219725)

[Dmitry Sidorov via llvm-commits]

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MrSidims wrote:

Correct. The users come in the follow-up that gates the AMDGPU V_FMA_MIX/V_MAD_MIX patterns on contract, which needs all three. I kept them here so that PR stays but I can move them there if you prefer defs to land with their first user.

https://github.com/llvm/llvm-project/pull/219725