cmd/compile: move decompose builtin closer to late opt

* Shaves about 10k from pkg/tools/linux_amd64.
* Was suggested by drchase before
* Found by looking at ssa output of #14758

Change-Id: If2c4ddf3b2603d4dfd8fb4d9199b9a3dcb05b17d
Reviewed-on: https://go-review.googlesource.com/20570Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -174,7 +174,6 @@ var passes = [...]pass{
 	{name: "early deadcode", fn: deadcode}, // remove generated dead code to avoid doing pointless work during opt
 	{name: "short circuit", fn: shortcircuit},
 	{name: "decompose user", fn: decomposeUser, required: true},
-	{name: "decompose builtin", fn: decomposeBuiltIn, required: true},
 	{name: "opt", fn: opt, required: true},           // TODO: split required rules and optimizing rules
 	{name: "zero arg cse", fn: zcse, required: true}, // required to merge OpSB values
 	{name: "opt deadcode", fn: deadcode},             // remove any blocks orphaned during opt
@@ -182,6 +181,7 @@ var passes = [...]pass{
 	{name: "phiopt", fn: phiopt},
 	{name: "nilcheckelim", fn: nilcheckelim},
 	{name: "prove", fn: prove},
+	{name: "decompose builtin", fn: decomposeBuiltIn, required: true},
 	{name: "dec", fn: dec, required: true},
 	{name: "late opt", fn: opt}, // TODO: split required rules and optimizing rules
 	{name: "generic deadcode", fn: deadcode},
@@ -234,7 +234,7 @@ var passOrder = [...]constraint{
 	{"generic deadcode", "tighten"},
 	{"generic cse", "tighten"},
 	// don't run optimization pass until we've decomposed builtin objects
-	{"decompose builtin", "opt"},
+	{"decompose builtin", "late opt"},
 	// don't layout blocks until critical edges have been removed
 	{"critical", "layout"},
 	// regalloc requires the removal of all critical edges