[dev.ssa] cmd/compile: remember names of values

For debugging, spill values to named variables instead of autotmp_
variables if possible.  We do this by keeping a name -> value map
for each function, keep it up-to-date during deadcode elim, and use
it to override spill decisions in stackalloc.

It might even make stack frames a bit smaller, as it makes it easy
to identify a set of spills which are likely not to interfere.

This just works for one-word variables for now.  Strings/slices
will be a separate CL.

Change-Id: Ie89eba8cab16bcd41b311c479ec46dd7e64cdb67
Reviewed-on: https://go-review.googlesource.com/16336
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -25,6 +25,13 @@ type Func struct {
 
 	// when register allocation is done, maps value ids to locations
 	RegAlloc []Location
+
+	// map from *gc.Node to set of Values that represent that Node.
+	// The Node must be an ONAME with PPARAM, PPARAMOUT, or PAUTO class.
+	NamedValues map[GCNode][]*Value
+	// Names is a copy of NamedValues.Keys.  We keep a separate list
+	// of keys to make iteration order deterministic.
+	Names []GCNode
 }
 
 // NumBlocks returns an integer larger than the id of any Block in the Func.