[AArch64][GlobalISel] Fix the G_EXTLOAD combiner creating non-extending illegal instructions.

This fixes loads like 's1 = load %p (load 1 from %p)' being combined with an
extend into an illegal 's8 = g_extload %p (load 1 from %p)' which doesn't do any
extension, by avoiding touching those < s8 size loads.

This bug was uncovered by a verifier update r351584, which I reverted it to keep
the bots green.

llvm-svn: 352311