The stlxr (store-release exclusive register) instructions in Arm64 have similar
restrictions to Arm's strex instructions - the status register must not alias
the source or address registers.

Enforce this in the assembler and simulator, and modify Turbofan and cctest to
conform to this. Also, make a small improvement to the code generated for
compare and exchange.

This is a port of 44c52f7b.

Bug: 
Change-Id: Ia3a8c39b09c5cb579357a5f61c3d88f13d61b724
Reviewed-on: https://chromium-review.googlesource.com/793037Reviewed-by: Ben Smith <binji@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#49714}