[AArch64][GlobalISel] Re-enable selection of volatile loads.

We previously disabled this in r323371 because of a bug where we selected an
extending load, but didn't delete the old G_LOAD, resulting in two loads being
generated for volatile loads.

Since we now have dedicated G_SEXTLOAD/G_ZEXTLOAD operations, and that the
tablegen patterns should no longer be able to select (ext(load x)) patterns, it
should be safe to re-enable it.

The old test case should still work as expected.

llvm-svn: 348320