[dev.simd] simd/_gen: add mem peephole with feat mismatches
This CL attempts to add peepholes for Op -> Opload where the Opload has
a different CPU feature than Op. However the new simdgen changes doesn't
do anything because such peepholes do not exist.
Change-Id: I20c3e4b43bb7414c3a309d77786218372ca1b5b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/711380 Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>