[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/compile.go

@@ -83,8 +83,8 @@ type pass struct {
 var passes = [...]pass{
 	{"phielim", phielim},
 	{"copyelim", copyelim},
-	{"decompose", decompose},
 	{"early deadcode", deadcode}, // remove generated dead code to avoid doing pointless work during opt
+	{"decompose", decompose},
 	{"opt", opt},
 	{"opt deadcode", deadcode}, // remove any blocks orphaned during opt
 	{"generic cse", cse},