[dev.ssa] cmd/compile: move tuple selectors to generator's block in CSE

CSE may substitute a tuple generator with another one in a different
block. In this case, since we want tuple selectors to stay together
with the tuple generator, copy the selector to the new generator's
block and rewrite its use.

Op.isTupleGenerator and Op.isTupleSelector are introduced to assert
tuple ops. Use it in tighten as well.

Updates #15365.

Change-Id: Ia9e8c734b9cc3bc9fca4a2750041eef9cdfac5a5
Reviewed-on: https://go-review.googlesource.com/24137
Reviewed-by: David Chase <drchase@google.com>

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

@@ -163,6 +163,29 @@ func cse(f *Func) {
 		}
 	}
 
+	// if we rewrite a tuple generator to a new one in a different block,
+	// copy its selectors to the new generator's block, so tuple generator
+	// and selectors stay together.
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			if rewrite[v.ID] != nil {
+				continue
+			}
+			if !v.Op.isTupleSelector() {
+				continue
+			}
+			if !v.Args[0].Op.isTupleGenerator() {
+				f.Fatalf("arg of tuple selector %s is not a tuple: %s", v.String(), v.Args[0].LongString())
+			}
+			t := rewrite[v.Args[0].ID]
+			if t != nil && t.Block != b {
+				// v.Args[0] is tuple generator, CSE'd into a different block as t, v is left behind
+				c := v.copyInto(t.Block)
+				rewrite[v.ID] = c
+			}
+		}
+	}
+
 	rewrites := int64(0)
 
 	// Apply substitutions