cmd/compile: fix typecheck range over rune literal

cmd/compile: fix typecheck range over rune literal

With range over int, the rune literal in range expression will be left
as untyped rune, but idealType is not handling this case, causing ICE.

Fixing this by setting the concrete type for untyped rune expresison.

Fixes #64471

Change-Id: I07a151c54ea1d9e1b92e4d96cdfb6e73dca13862
Reviewed-on: https://go-review.googlesource.com/c/go/+/546296
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>