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>

test/codegen/slices.go

@@ -6,6 +6,8 @@
 
 package codegen
 
+import "unsafe"
+
 // This file contains code generation tests related to the handling of
 // slice types.
 
@@ -368,3 +370,16 @@ func SliceWithSubtractBound(a []int, b int) []int {
 	// ppc64:"SUBC",-"NEG"
 	return a[(3 - b):]
 }
+
+// --------------------------------------- //
+//   Code generation for unsafe.Slice      //
+// --------------------------------------- //
+
+func Slice1(p *byte, i int) []byte {
+	// amd64:-"MULQ"
+	return unsafe.Slice(p, i)
+}
+func Slice0(p *struct{}, i int) []struct{} {
+	// amd64:-"MULQ"
+	return unsafe.Slice(p, i)
+}