runtime: short-circuit typedmemmove when dst==src

Change-Id: I855268a4c0d07ad602ec90f5da66422d3d87c5f2
Reviewed-on: https://go-review.googlesource.com/94595
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/mbarrier.go

@@ -154,6 +154,9 @@ import (
 //
 //go:nosplit
 func typedmemmove(typ *_type, dst, src unsafe.Pointer) {
+	if dst == src {
+		return
+	}
 	if typ.kind&kindNoPointers == 0 {
 		bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.size)
 	}