AMDGPU: Fix DPP combiner using isOperandLegal on incomplete inst (#155595) It is not safe to use isOperandLegal on an instruction that does not have a complete set of operands. Unforunately the APIs are not set up in a convenient way to speculatively check if an instruction will be legal in a hypothetical instruction. Build all the operands and then verify they are legal after. This is clumsy, we should have a more direct check for will these operands give a legal instruction. This seems to fix a missed optimization in the gfx11 test. The fold was firing for gfx1150, but not gfx1100. Both should support vop3 literals so I'm not sure why it wasn't working before.
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