cmd/compile: only use dictionaries for conversions to type parameters

Conversions to regular concrete types should not be rewritten during
stenciling.

Fixes #47740

Change-Id: I2b45e22f962dcd2e18bd6cc876ebc0f850860822
Reviewed-on: https://go-review.googlesource.com/c/go/+/342989


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>