[dev.ssa] cmd/compile: optimize phi ops

Redo how we keep track of forward references when building SSA.
When the forward reference is resolved, update the Value node
in place.

Improve the phi elimination pass so it can simplify phis of phis.

Give SSA package access to decoded line numbers.  Fix line numbers
for constant booleans.

Change-Id: I3dc9896148d260be2f3dd14cbe5db639ec9fa6b7
Reviewed-on: https://go-review.googlesource.com/18674
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Keith Randall <khr@golang.org>

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

@@ -81,8 +81,9 @@ type pass struct {
 
 // list of passes for the compiler
 var passes = [...]pass{
-	{"phielim", phielim},
-	{"copyelim", copyelim},
+	// TODO: combine phielim and copyelim into a single pass?
+	{"early phielim", phielim},
+	{"early copyelim", copyelim},
 	{"early deadcode", deadcode}, // remove generated dead code to avoid doing pointless work during opt
 	{"decompose", decompose},
 	{"opt", opt},
@@ -97,6 +98,9 @@ var passes = [...]pass{
 	{"lowered cse", cse},
 	{"lowered deadcode", deadcode},
 	{"checkLower", checkLower},
+	{"late phielim", phielim},
+	{"late copyelim", copyelim},
+	{"late deadcode", deadcode},
 	{"critical", critical},   // remove critical edges
 	{"layout", layout},       // schedule blocks
 	{"schedule", schedule},   // schedule values