[dev.ssa] cmd/compile: add decompose pass

Decompose breaks compound objects up into pieces that can be
operated on by the target architecture.  The decompose pass only
does phi ops, the rest is done by the rewrite rules in generic.rules.

Compound objects include strings,slices,interfaces,structs,arrays.

Arrays aren't decomposed because of indexing (we could support
constant indexes, but dynamic indexes can't be handled using SSA).
Structs will come in a subsequent CL.

TODO: after this pass we have lost the association between, e.g.,
a string's pointer and its size.  It would be nice if we could keep
that information around for debugging info somehow.

Change-Id: I6379ab962a7beef62297d0f68c421f22aa0a0901
Reviewed-on: https://go-review.googlesource.com/13683
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -60,6 +60,7 @@ 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
 	{"opt", opt},
 	{"opt deadcode", deadcode}, // remove any blocks orphaned during opt
@@ -103,6 +104,8 @@ var passOrder = [...]constraint{
 	// tighten will be most effective when as many values have been removed as possible
 	{"generic deadcode", "tighten"},
 	{"generic cse", "tighten"},
+	// don't run optimization pass until we've decomposed compound objects
+	{"decompose", "opt"},
 	// don't layout blocks until critical edges have been removed
 	{"critical", "layout"},
 	// regalloc requires the removal of all critical edges