[AMDGPU][CostModel] Improve cost estimation for fused {fadd|fsub}(a,fmul(b,c))

Summary:
If result of fmul(b,c) has one use, in almost all cases (except denormals are
IEEE) the pair of operations will be fused in one fma/mad/mac/etc.

Reviewers: rampitec

Reviewed By: rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83919