[AArch64] Load/store optimizer fixes and cleanup.

This includes a couple of fixes after #71908 for bundles and some cleanup for
the debug output. One was an iterator type that asserted on bundles, the second
a rather subtle issue where forAllMIsUntilDef would hit the LdStLimit when
renaming registers, meaning the last instruction was not updated leaving an
invalid `ldp x6, x6` instruction.