cmd/compile: fix the names of methods created during type substitution

cmd/compile: fix the names of methods created during type substitution

The names given to methods of types created during type substitution
were possible incorrect when the type parameters themselves were nested
types.

Fixes #50485

Change-Id: I7e0043ed22c26406a5f9d8d51d9e928770a678f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/377494
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>