[dev.simd] cmd/compile: first semi-working "midway" translator

This is a source-to-source translator, ported into
the compiler front-end.  The strategy for correctly
checking types and scopes is to re-run type checking.

Includes emulations and tests.  This is actually
tiny-highway v0.

Change-Id: Ifd354035894e43812d6134e856baab72626788ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/763100
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>