cmd/compile/internal/ssa: update BlockKind documentation

BlockCall was removed in golang.org/cl/28950.

Change-Id: Ib8d9f3111bf3dc01956dd776afeb345ede8bc933
Reviewed-on: https://go-review.googlesource.com/30353Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/ssa/block.go

@@ -98,7 +98,7 @@ func (e Edge) Index() int {
 //     Exit        return mem                []
 //    Plain               nil            [next]
 //       If   a boolean Value      [then, else]
-//     Call               mem  [nopanic, panic]  (control opcode should be OpCall or OpStaticCall)
+//    Defer               mem  [nopanic, panic]  (control opcode should be OpDeferCall)
 type BlockKind int8
 
 // short form print