cmd/compile: early devirtualization of interface method calls

After inlining, add a pass that looks for interface calls where we can
statically determine the interface value's concrete type. If such a
case is found, insert an explicit type assertion to the concrete type
so that escape analysis can see it.

Fixes #33160.

Change-Id: I36932c691693f0069e34384086d63133e249b06b
Reviewed-on: https://go-review.googlesource.com/c/go/+/264837
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/main.go

@@ -701,6 +701,13 @@ func Main(archInit func(*Arch)) {
 		})
 	}
 
+	for _, n := range xtop {
+		if n.Op == ODCLFUNC {
+			devirtualize(n)
+		}
+	}
+	Curfn = nil
+
 	// Phase 6: Escape analysis.
 	// Required for moving heap allocations onto stack,
 	// which in turn is required by the closure implementation,