[AMDGPU][ConstantFolding] Fold llvm.amdgcn.fmul.legacy intrinsic

Reviewers: arsenm, rampitec, nhaehnle

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74835