[mlir] Clean-up math -> libm/llvm conversion.

At the moment, there is an optional log1pBenefit
populateMathToLibmConversionPatterns which is used to increase the priority of
the log1p->libm pattern compared to log1p->llvm pattern that approximates
log1p with precision issues. Instead, we can have a flag for the MathToLLVM
pass to enable or disable the imprecise approximation.

Differential Revision: https://reviews.llvm.org/D144450