cmd/compile: optimize unsafe.Slice generated code

cmd/compile: optimize unsafe.Slice generated code

We don't need a multiply when the element type is size 0 or 1.

The panic functions don't return, so we don't need any post-call
code (register restores, etc.).

Change-Id: I0dcea5df56d29d7be26554ddca966b3903c672e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/419754
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>