[mlir][math] Added math::FPowI conversion to calls of outlined implementations.

Power functions are implemented as linkonce_odr scalar functions
for FPowI operations met in a module.
Vector form of FPowI is linearized into a sequence of calls
of the scalar functions.

Option {min-width-of-fpowi-exponent} controls which FPowI operations
are converted by MathToFuncs: if the width of the exponent's integer
type is less than the specified value, then the operation is not converted.

Flang will specify {min-width-of-fpowi-exponent=33} to make sure that
math::FPowI operations with exponent wider than 32 bits will be converted
by MathToFuncs, and operations with more narrow exponent will be left
for MathToLLVM to convert them to LLVM::PowIOp.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D139804