cmd/compile: fix checkptr false positive for (*[Big]T)(ptr)[:n:n] pattern

The checkptr instrumentation is currently inserted before slice
operation has validated that n <= Big. So instead of panic, checkptr
have false positive throws.

To fix this, just insert the checkptr instrumentation after the bound
checking during SSA generation.

Fixes #46938

Change-Id: I9dbf84441c711842ccc883f3654ca8766ac696d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/343972


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>