[RISCV] Require alignment when forming gather with larger element type

This fixes a bug in my 928564ca that didn't get noticed in review.  I found it when looking at the strided load case (upcoming patch), and realized the previous commit was buggy too.

p.s. Sorry for the slightly confusing test diff.  I'd apparently used the wrong mask for the aligned positive test; it was actually unaligned.  Didn't seem worthy of a separate precommit.