go.tools/ssa: fix bug: lifting pass invalidated Referrers info for DebugRef's X operand.

Tested: by enabling debug info globally in stdlib_test,
the existing sanity checks catch it.

R=gri
CC=golang-codereviews
https://golang.org/cl/50570043

ssa/lift.go

@@ -523,6 +523,11 @@ func rename(u *BasicBlock, renaming []Value, newPhis newPhiMap) {
 				if instr.IsAddr {
 					instr.X = renamed(renaming, alloc)
 					instr.IsAddr = false
+
+					// Add DebugRef to instr.X's referrers.
+					if refs := instr.X.Referrers(); refs != nil {
+						*refs = append(*refs, instr)
+					}
 				} else {
 					// A source expression denotes the address
 					// of an Alloc that was optimized away.

ssa/stdlib_test.go

@@ -23,7 +23,7 @@ import (
 	"code.google.com/p/go.tools/ssa/ssautil"
 )
 
-const debugMode = false
+const debugMode = true // cost: +30% space, +18% time for SSA building
 
 func allPackages() []string {
 	var pkgs []string