[Flang][HLFIR] Intrinsics: Propagate fast math flags

Add a new FirOpBuilder constructor to propagate the fast math flag
from an operation. Use this constructor in the LowerHLFIRIntrinsics
pass.

This fixes the performance issue with the hlfir intrinsics flow
for polyhedron/test_fpu2.

Reviewed By: tblah, vzakhari

Differential Revision: https://reviews.llvm.org/D155438