cmd/compile: make sure that the names created for instantiated type are the same

Now we have two functions that create names for instantiated types.
They are inconsistent when dealing with byte/rune type.

This CL makes instTypeName2 reuse the code of typecheck.InstTypeName

Fixes #48198

Change-Id: I4c216b532cba6618ef9b63fd0b76e8f1c0ed7a75
Reviewed-on: https://go-review.googlesource.com/c/go/+/347491


Reviewed-by: Dan Scales <danscales@google.com>
Trust: Dan Scales <danscales@google.com>
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>