Tighten lax vector-conversion rules and enforce them consistently.

When a lax conversion featured a vector and a non-vector, we were
only requiring the non-vector to be a scalar type, but really it
needs to be a real type (i.e. integral or real floating); it is
not reasonable to allow a pointer, member pointer, or complex
type here.

r198474 required lax conversions to match in "data size", i.e.
element size * element count, forbidding matches that happen
only because a vector is rounded up to the nearest power of two
in size.  Unfortunately, the erroneous logic was repeated in
several different places; unify them to use the new condition,
so that it triggers for arbitrary conversions and not just
those performed as part of binary operator checking.

rdar://15931426