[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/html.go

@@ -472,3 +472,7 @@ func (p htmlFuncPrinter) startDepCycle() {
 func (p htmlFuncPrinter) endDepCycle() {
 	fmt.Fprintln(p.w, "</span>")
 }
+
+func (p htmlFuncPrinter) named(n LocalSlot, vals []*Value) {
+	// TODO
+}