AArch64: More correctly constrain target vector extend lowering.

The AArch64 target lowering for [zs]ext of vectors is set up to handle
input simple types and expects the generic SDag path to do something reasonable
with anything that's not a simple type. The code, however, was only
checking that the result type was a simple type and assuming that
implied that the source type would also be a simple type. That's not a
valid assumption, as operations like "zext <1 x i1> %0 to <1 x i32>"
demonstrate. The fix is to simply explicitly validate the source type
as well as the result type.

PR20791