cmd/compile: check that SSA memory args are in the right place

Fixes #15510

Change-Id: I2e0568778ef90cf29712753b8c42109ef84a0256
Reviewed-on: https://go-review.googlesource.com/22784
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -208,10 +208,16 @@ func checkFunc(f *Func) {
 				f.Fatalf("value %s has an AuxInt value %d but shouldn't", v.LongString(), v.AuxInt)
 			}
 
-			for _, arg := range v.Args {
+			for i, arg := range v.Args {
 				if arg == nil {
 					f.Fatalf("value %s has nil arg", v.LongString())
 				}
+				if v.Op != OpPhi {
+					// For non-Phi ops, memory args must be last, if present
+					if arg.Type.IsMemory() && i != len(v.Args)-1 {
+						f.Fatalf("value %s has non-final memory arg (%d < %d)", v.LongString(), i, len(v.Args)-1)
+					}
+				}
 			}
 
 			if valueMark[v.ID] {