Ext mul's codegen assumes that all inputs are in registers, but the
instruction-selector wasn't the correct constraints. The codegen for ext
mul is slightly complicated so we chose to restrict the inputs to be
registers rather than changing codegen.

Bug: chromium:1165966,v8:11262
Change-Id: I5d4eb56d17a4d0a2927b089dbf74362c7e7ff4fc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2626711Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72073}