cmd/compile/internal/gc: treat cap/len as safe in mayAffectMemory

OLEN and OCAP can't affect memory state as long as their
arguments don't.

Re-organized case bodies to avoid duplicating same branches for
recursive invocations.

Change-Id: I30407143429f7dd1891badb70df88969ed267535
Reviewed-on: https://go-review.googlesource.com/133555
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -689,18 +689,16 @@ func (e *EscState) mayAffectMemory(n *Node) bool {
 	switch n.Op {
 	case ONAME, OCLOSUREVAR, OLITERAL:
 		return false
-	case ODOT, ODOTPTR:
-		return e.mayAffectMemory(n.Left)
-	case OIND, OCONVNOP:
-		return e.mayAffectMemory(n.Left)
-	case OCONV:
-		return e.mayAffectMemory(n.Left)
-	case OINDEX:
-		return e.mayAffectMemory(n.Left) || e.mayAffectMemory(n.Right)
-	case OADD, OSUB, OOR, OXOR, OMUL, OLSH, ORSH, OAND, OANDNOT, ODIV, OMOD:
+
+	// Left+Right group.
+	case OINDEX, OADD, OSUB, OOR, OXOR, OMUL, OLSH, ORSH, OAND, OANDNOT, ODIV, OMOD:
 		return e.mayAffectMemory(n.Left) || e.mayAffectMemory(n.Right)
-	case ONOT, OCOM, OPLUS, OMINUS, OALIGNOF, OOFFSETOF, OSIZEOF:
+
+	// Left group.
+	case ODOT, ODOTPTR, OIND, OCONVNOP, OCONV, OLEN, OCAP,
+		ONOT, OCOM, OPLUS, OMINUS, OALIGNOF, OOFFSETOF, OSIZEOF:
 		return e.mayAffectMemory(n.Left)
+
 	default:
 		return true
 	}