[dev.simd] cmd/compile: support simd(imm,fp) returns gp

[dev.simd] cmd/compile: support simd(imm,fp) returns gp

These changes are required to make gp-returning simd
ops work.  amd64/ssa.go includes a new code generator
helper, gc/main.go initializes intrinsics AFTER
the types, ssa/_gen/*AMD64.go add another register
shape to the simd ops function.

This CL should be submitted after simdgen CL 683858
which generated some of the changes.

Change-Id: I0af752ba8882fa131b875ff9c741ef70afbc60d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/683816
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>