[DAG][AArch64] Handle vscale addressing modes in...

[DAG][AArch64] Handle vscale addressing modes in reassociationCanBreakAddressingModePattern (#89908)

reassociationCanBreakAddressingModePattern tries to prevent bad add
reassociations that would break adrressing mode patterns. This adds
support for vscale offset addressing modes, making sure we don't break
patterns that already exist. It does not optimize _to_ the correct
addressing modes yet, but prevents us from optimizating _away_ from
them.