[dev.ssa] cmd/compile: start arguments as spilled

Declare a function's arguments as having already been
spilled so their use just requires a restore.

Allow spill locations to be portions of larger objects the stack.
Required to load portions of compound input arguments.

Rename the memory input to InputMem.  Use Arg for the
pre-spilled argument values.

Change-Id: I8fe2a03ffbba1022d98bfae2052b376b96d32dda
Reviewed-on: https://go-review.googlesource.com/16536
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -26,12 +26,11 @@ 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
+	// map from LocalSlot to set of Values that we want to store in that slot.
+	NamedValues map[LocalSlot][]*Value
 	// Names is a copy of NamedValues.Keys.  We keep a separate list
 	// of keys to make iteration order deterministic.
-	Names []GCNode
+	Names []LocalSlot
 }
 
 // NumBlocks returns an integer larger than the id of any Block in the Func.