GlobalISel: Add memSizeNotByteSizePow2 legality helper

This is really a replacement for memSizeInBytesNotPow2 that actually
does what most every target wants. In particular, since s1 rounds to 1
byte, it wasn't lowered by this predicate. This results in targets
needing to think harder and add more matchers to catch all the
degenerate cases.

Also small bug fix that prevented the correct insertion of
G_ASSERT_ZEXT in the AArch64 use case.