[dev.simd] cmd/compile: removed a map iteration from generator; tweaked type order

Output of CL 682316

Change-Id: I566486085fbd8a5437a5904ed02f718da7fed2c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/682355
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: dr2chase