This CL avoids redundant loads of the instance from the frame by caching
it in a register if possible. This register will be the first one to be
cleared once we run out of registers (hence it's called a "volatile
register"). On local tests, this seems to reduce most redundant loads
within a function, and it also reduces the load for the stack check in
the function prologue.
After the stack check, we need to discard the cached instance though,
since the potential runtime call for the stack check might clobber it.
This will be addressed in a follow-up CL by re-loading the cached
instance after the stack check. This is expected to remove another good
chunk of instance loads, because the instance would initially be
available in a register when starting the function code.

R=thibaudm@chromium.org

Bug: v8:11336
Change-Id: Ie65ab81263fb9d972f4b7a6daaef86cf704874ef
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2695401
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72779}