[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/gc/pgen.go

@@ -484,6 +484,9 @@ func compile(fn *Node) {
 
 	if ssafn != nil && usessa {
 		genssa(ssafn, ptxt, gcargs, gclocals)
+		if Curfn.Func.Endlineno != 0 {
+			lineno = Curfn.Func.Endlineno
+		}
 		return
 	}
 	Genlist(Curfn.Func.Enter)