[flang] Fix crash in folding of DPROD() with non-scalar arguments

DPROD(x,y) is defined as DBLE(x)*DBLE(y) and that's exactly how
the implementation of its rewriting and possible folding should
be implemented, instead of the current code that only works when
both arguments are scalar and crashes otherwise.

Fixes https://github.com/llvm/llvm-project/issues/63991.

Differential Revision: https://reviews.llvm.org/D156754