cmd/compile: ignore some dead code during escape analysis

This is the escape analysis analog of CL 37499.

Fixes #12397
Fixes #16871

The only "moved to heap" decisions eliminated by this
CL in std+cmd are:

cmd/compile/internal/gc/const.go:1514: moved to heap: ac
cmd/compile/internal/gc/const.go:1515: moved to heap: bd
cmd/compile/internal/gc/const.go:1516: moved to heap: bc
cmd/compile/internal/gc/const.go:1517: moved to heap: ad
cmd/compile/internal/gc/const.go:1546: moved to heap: ac
cmd/compile/internal/gc/const.go:1547: moved to heap: bd
cmd/compile/internal/gc/const.go:1548: moved to heap: bc
cmd/compile/internal/gc/const.go:1549: moved to heap: ad
cmd/compile/internal/gc/const.go:1550: moved to heap: cc_plus
cmd/compile/internal/gc/export.go:162: moved to heap: copy
cmd/compile/internal/gc/mpfloat.go:66: moved to heap: b
cmd/compile/internal/gc/mpfloat.go:97: moved to heap: b

Change-Id: I0d420b69c84a41ba9968c394e8957910bab5edea
Reviewed-on: https://go-review.googlesource.com/37508Reviewed-by: David Chase <drchase@google.com>

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

@@ -685,11 +685,20 @@ func (e *EscState) esc(n *Node, parent *Node) {
 		e.escassignSinkWhy(n, n, "too large for stack") // TODO category: tooLarge
 	}
 
+	if n.Op == OIF && Isconst(n.Left, CTBOOL) {
+		// Don't examine dead code.
+		if n.Left.Bool() {
+			e.esclist(n.Nbody, n)
+		} else {
+			e.esclist(n.Rlist, n)
+		}
+	} else {
 		e.esc(n.Left, n)
 		e.esc(n.Right, n)
 		e.esclist(n.Nbody, n)
 		e.esclist(n.List, n)
 		e.esclist(n.Rlist, n)
+	}
 
 	if n.Op == OFOR || n.Op == ORANGE {
 		e.loopdepth--

test/escape2.go

@@ -1824,3 +1824,18 @@ func issue11387(x int) func() int {
 	copy(slice2, slice1)
 	return slice2[0]
 }
+
+func issue12397(x, y int) { // ERROR "moved to heap: y$"
+	// x does not escape below, because all relevant code is dead.
+	if false {
+		gxx = &x
+	} else {
+		gxx = &y // ERROR "&y escapes to heap$"
+	}
+
+	if true {
+		gxx = &y // ERROR "&y escapes to heap$"
+	} else {
+		gxx = &x
+	}
+}