[dev.simd] cmd/compile, simd: jump table for imm ops

[dev.simd] cmd/compile, simd: jump table for imm ops

This CL fixes some errors in prog generation for imm operations, please
see the changes in ssa.go for details.

This CL also implements the jump table for non-const immediate arg. The
current implementation exhaust 0-255, the bound-checked version will be
in the next CL.

This CL is partially generated by CL 694375.

Change-Id: I75fe9900430b4fca5b39b0c0958a13b20b1104b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/694395
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>