[flang] Change return type of reduction intrinsics

Comments in the recent patch https://reviews.llvm.org/D149964,
mentioned that using hlfir_ExprType in cases where intrinsics
return simple scalars adds unnecessary abstraction that isn't
needed unless an array type is being used.

This patch modifies the HLFIR operations for product, sum and any
so that they only return a hlfir_ExprType when the result is an array,
otherwise they will return just the simple scalar type.

Differential Revision: https://reviews.llvm.org/D150877