[release-branch.go1.20] cmd/compile: don't set range expr key/value type if already set

[release-branch.go1.20] cmd/compile: don't set range expr key/value type if already set

Unified IR already records the correct type for them.

Fixes #59450

Change-Id: I275c45b48f67bde55c8e2079d60b5868d0acde7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/481555
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/482655
Auto-Submit: Michael Knyszek <mknyszek@google.com>