[AArch64] Remove custom zext/sext legalization code.

Currently performExtendCombine assumes that the src-element bitwidth * 2
is a valid MVT. But this is not the case for i1 and it causes a crash on
the v64i1 test cases added in this patch.

It turns out that this code appears to not be needed; the same patterns are
handled by other code and we end up with the same results, even without the
custom lowering. I also added additional test cases in a50037aa.

Let's just remove the unneeded code.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D99437