[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/regalloc.go

@@ -759,6 +759,16 @@ func (s *regAllocState) regalloc(f *Func) {
 				pc++
 				continue
 			}
+			if v.Op == OpArg {
+				// Args are "pre-spilled" values.  We don't allocate
+				// any register here.  We just set up the spill pointer to
+				// point at itself and any later user will restore it to use it.
+				s.values[v.ID].spill = v
+				s.values[v.ID].spillUsed = true // use is guaranteed
+				b.Values = append(b.Values, v)
+				pc++
+				continue
+			}
 			s.clearUses(pc*2 - 1)
 			regspec := opcodeTable[v.Op].reg
 			if regDebug {