[flang][hlfir] lower hlfir.product into fir runtime call

The shared code for lowering the sum and product operations in
flang/lib/Optimizer/HLFIR/Transforms/LowerHLFIRIntrinsics.cpp have
been moved into a new class HlfirReductionIntrinsicConverion.

Depends on: D148719

Differential Revision: https://reviews.llvm.org/D149644