[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/deadcode.go

@@ -162,6 +162,25 @@ func deadcode(f *Func) {
 	}
 	f.Blocks = f.Blocks[:i]
 
+	// Remove dead entries from namedValues map.
+	for name, values := range f.NamedValues {
+		i := 0
+		for _, v := range values {
+			for v.Op == OpCopy {
+				v = v.Args[0]
+			}
+			if live[v.ID] {
+				values[i] = v
+				i++
+			}
+		}
+		f.NamedValues[name] = values[:i]
+		tail := values[i:]
+		for j := range tail {
+			tail[j] = nil
+		}
+	}
+
 	// TODO: renumber Blocks and Values densely?
 	// TODO: save dead Values and Blocks for reuse?  Or should we just let GC handle it?
 }