[ArgPromotion] Make a non-byval promotion attempt first

It makes sense to make a non-byval promotion attempt first and then
fall back to the byval one. The non-byval ('usual') promotion is
generally better, for example it does promotion even when a structure
has more elements than 'MaxElements' but not all of them are actually
used in the function.

Differential Revision: https://reviews.llvm.org/D124514