[AArch64] Also combine vector selects fed by non-i1 SETCCs.

After legalization, scalar SETCC has an i32 result type on AArch64.
The i1 requirement seems too conservative, replace it with an assert.

This also means that we now can run after legalization. That should also
be fine, since the ops legalizer runs again after each combine, and
all types created all have the same sizes as the (legal) inputs.

Exposed by r235917; while there, robustize its tests (bsl also uses the
register it defines).

llvm-svn: 235922