[release-branch.go1.21] cmd/compile: make backingArrayPtrLen to return typecheck...

[release-branch.go1.21] cmd/compile: make backingArrayPtrLen to return typecheck-ed nodes

Fixes #61909

Change-Id: Ief8e3a6c42c0644c9f71ebef5f28a294cd7c153f
Reviewed-on: https://go-review.googlesource.com/c/go/+/517936
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/518115
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>