cmd/compile: fix arg to getcallerpc

getcallerpc's arg needs to point to the first argument slot.
I believe this bug was introduced by Michel's itab changes
(specifically https://go-review.googlesource.com/c/20902).

Fixes #15145

Change-Id: Ifb2e17f3658e2136c7950dfc789b4d5706320683
Reviewed-on: https://go-review.googlesource.com/21931Reviewed-by: Michel Lespinasse <walken@google.com>

src/runtime/iface.go

@@ -173,7 +173,7 @@ func convT2E(t *_type, elem unsafe.Pointer, x unsafe.Pointer) (e eface) {
 func convT2I(tab *itab, elem unsafe.Pointer, x unsafe.Pointer) (i iface) {
 	t := tab._type
 	if raceenabled {
-		raceReadObjectPC(t, elem, getcallerpc(unsafe.Pointer(&t)), funcPC(convT2I))
+		raceReadObjectPC(t, elem, getcallerpc(unsafe.Pointer(&tab)), funcPC(convT2I))
 	}
 	if msanenabled {
 		msanread(elem, t.size)