cmd/compile,runtime: open code unsafe.Slice

cmd/compile,runtime: open code unsafe.Slice

So prevent heavy runtime call overhead, and the compiler will have a
chance to optimize the bound check.

With this optimization, changing runtime/stack.go to use unsafe.Slice
no longer negatively impacts stack copying performance:

name                   old time/op    new time/op    delta
StackCopyWithStkobj-8    16.3ms ± 6%    16.5ms ± 5%   ~     (p=0.382 n=8+8)

name                   old alloc/op   new alloc/op   delta
StackCopyWithStkobj-8     17.0B ± 0%     17.0B ± 0%   ~     (all equal)

name                   old allocs/op  new allocs/op  delta
StackCopyWithStkobj-8      1.00 ± 0%      1.00 ± 0%   ~     (all equal)

Fixes #48798

Change-Id: I731a9a4abd6dd6846f44eece7f86025b7bb1141b
Reviewed-on: https://go-review.googlesource.com/c/go/+/362934
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>