cmd/compile: fix conv of slice of user-define byte type to string

types2 allows the conversion of a slice of a user-defined byte type B
(not builtin uint8 or byte) to string. But runtime.slicebytetostring
requires a []byte argument, so add in a CONVNOP from []B to []byte if
needed. Same for the conversion of a slice of user-defined rune types to
string.

I made the same change in the transformations of the old typechecker, so
as to keep tcConv() and transformConv() in sync. That fixes the bug for
-G=0 mode as well.

Fixes #23536

Change-Id: Ic79364427f27489187f3f8015bdfbf0769a70d69
Reviewed-on: https://go-review.googlesource.com/c/go/+/376056


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
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>