[dev.simd] cmd/compile: remove fused mul/add/sub shapes.

[dev.simd] cmd/compile: remove fused mul/add/sub shapes.

This CL only keeps one shape of those fused mul/add/sub operations. The
rest of the instructions will be generated during lowering as an
optimization.

This CL is generated by CL 682436.

Change-Id: Iadee1786185289838e04f3aa8f333844cfacc02e
Reviewed-on: https://go-review.googlesource.com/c/go/+/682435
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>