The codegen for q15 rounding mul and dot incorrectly assumes that the
second operand is always a register.

For dot, we change the codegen to accept an Operand.

For q15, we change the instruction-selector to always use register.

Bug: v8:10971,v8:10993
Change-Id: I30a421de47ba61693ca73e616bcbc27ab01ac7bc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2626712Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72180}