[AMDGPU] Convert mul24 sdwa detection to tablegen predicates

#193033 allowed detection of operands to a mul24 node that would probably fold using SDWA patterns to prevent folding to mad24 (which can't use SDWA) - but its SelectionDAG-only.

This ticket tracks conversion to a purely-tablegen predicate to allow us to remove the AMDGPUDAGToDAGISel::isSDWAOperand hook and support globalisel as well (with suitable additional test coverage).