This is the purpose of all the serie of patches: inline sum and product.
Patches 62..65, possibly don't deserve a patch of their own, they just make
the changes in gfc_conv_intrinsic_arith easier to read.

The combined diff (context, ignoring indent changes) also attached here.

Patch 62: Change conditions.
Patch 63: Update argument handling.
Patch 64: Change loop usage.
Patch 65: Change gfc_se structs initializers.
Patch 66: Inline sum.