[mlir][math]Expand Fused math.fmaf to a multiply-add

[mlir][math]Expand Fused math.fmaf to a multiply-add

Fused multiply and add are being pushed directly to the libm. This is problematic
for situations where libm is not available. This patch will break down a fused multiply and
add into a multiply followed by an add.

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D147811