AArch64: Allow rebasing pointers to make more use of stp/ldp

[david-arm]

If the disp (offset) in a Vptr is negative and outside the range of stp/ldp, then it's also outside the range of str/stur/ldr/ldur. This will force the offset to be materialised in a register and to then use the reg+reg addressing mode for the loads and stores. We'll end up with assembly like this:

mov x2, -1024 str x0, [x29, x2] mov x2, -1032 str x1, [x29, x2]

If the immediate can easily be encoded into an add or sub instruction, then we can rebase the pointer and it will still be worth it. We'll then end up with assembly like this:

add x2, x29, -1024 stp x0, x1, [x2]

which still saves two instructions.

[david-arm]

NOTE: This PR includes https://github.com/facebook/hhvm/pull/9676, so should probably wait for that to land first.

[meta-codesync[bot]]

@facebook-github-bot has imported this pull request. If you are a Meta employee, you can view this in D88008213. (Because this pull request was imported automatically, there will not be any future comments.)