[dev.simd] cmd/compile: exclude simd vars from merge local
It looks like mergelocals pass's liveness analysis does not handle simd
variables well.
The added test forces two vectors to spill in a way that does not work
with mergelocals: if the added check is removed, then `v` and `m` will
be marked merged and spilled to the same location, failing the test.
Change-Id: Ife4e4e939565d817fc24f7180cb791a5084dd191
Reviewed-on: https://go-review.googlesource.com/c/go/+/687375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com>