cmd/compile: use more lenient type inference for untyped arguments for go1.21

cmd/compile: use more lenient type inference for untyped arguments for go1.21

This CL permanently enables the new behavior for -lang=go1.21 and
newer, and keeps the existing behavior if -lang=go1.20 or older.

To be submitted once #58671 is accepted.

For #58671.

Change-Id: I83a1d393f0ce7871be8f38ec35742d393946c55f
Reviewed-on: https://go-review.googlesource.com/c/go/+/496035
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>