Simplify binop_promote

binop_promote currently only handles integer sizes up to
builtin_long_long.  However, this may not handle 128-bit types.
Simplify this code, unify the C and non-C (but not OpenCL, as I don't
know how to test this) cases, and handle 128-bit integers as well.

This still doesn't exactly follow C or C++ rules.  This could be
implemented, but if so, I think it makes more sense as a C-specific
expression node.