cmd/compile/internal/typecheck: simplify tcAppend
cmd/compile/internal/typecheck: simplify tcAppend

After CL 418514, Unified IR now always write concrete type of constant
expressions, thus we don't need to handle untyped string anymore.

Change-Id: Ic930ee2418746ca9f4a5122c614e077bef8d7eb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/422041
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go