Liftoff can currently run out of registers when compiling an atomic
compare-exchange instruction. In order to see this crash, the following
conditions must be met:

- The offset in the instruction doesn't fit in a 12-bit immediate
- Either FLAG_untrusted_code_mitigations is false, or trap handlers are
  enabled, so that AddMemoryMasking decides to do nothing

The fix proposed in this CL is just to defer allocation of a temporary
register until after CalculateActualAddress has finished, because it
might have also needed a temporary register.

Change-Id: I28225614dcdbe2bcc9e52208f1e806baac89c5f1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2488840
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70687}