An improvement to generic shuffle improvement
(https://crrev.com/c/2152853) required a temporary SIMD register to hold
the mask, rather than pushing it onto a stack. The temporary register
requires that we UseUniqueRegister on the inputs, to prevent aliasing,
as we will write to the temp. However, we only need this for the generic
shuffle. We accidentally over-constraint all other pattern matched
shuffles, since they don't use any temps.

On a ~2000 line function containing ~150 shuffles (not all of which are
generic shuffles), we get 16 less instruction in the native code, and
actually see a very small improvement in the overall benchmarks.

Bug: v8:11270
Change-Id: I09974f7615e4b8f5e2416ed17ca47cc7613fd6b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2591857Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71818}