[flang] (NFC) Break up flang/runtime/reduction.cpp

The single source file reduction.cpp is a little large in
terms of both source lines and generated text bytes, so
split SUM, PRODUCT, FINDLOC, and MAXLOC/MAXVAL/MINLOC/MINVAL
off into their own C++ source files that share a set of
implementation function templates now in a common header.

Differential Revision: https://reviews.llvm.org/D101111