[flang] Run MathToLLVM conversion as part of math conversion PM.

We used to specify benefit=0 for MathToLibm patterns so that
MathToLLVM patterns are more preferable. D144450 removed
the benefit parameter, so we have to run MathToLLVM separately now.

This resolves Intrinsics/math-codegen failure caused by D144450.

Differential Revision: https://reviews.llvm.org/D144523