With a write barrier, stores with negative offsets would allocate a temporary
register to hold the offset when the `str` instruction is able to encode it.

For instance, when writing the object map:

```
;; This could be 'str x2, [x5, #-1]'
movn x4, #0x0
str x2, [x5, x4]
and x16, x5, #0xfffffffffffc0000
ldr x16, [x16, #8]
tbnz w16, #2, #+0xba8  ; Jump out-of-line
```

The reason behind this is that the out-of-line code uses an 'add' instruction on
the offset to compute the field address, putting pressure on the instruction
selector to make sure the immediate fits in both 'str' and 'add'.

But, this is not necessary since the macro-assembler is able to turn the 'add'
into a 'sub' or use a temporary register if needed.

Change-Id: I8838e4b81a0c0c1f90aa3d67861a9da1a6dfed06
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1708471Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#62803}