cmd/compile: `sccp` spends a lot of it's time optimizing deadcode

[Jorropo]

While looking at reordering the sccp pass I was trying to see what kind of hits it gets and what happens with them.

After looking 5 "random" findings from go build -a -gcflags=-d=ssa/sccp/debug=1 std I find that all of them are deadcode.
So I've added this diff:

diff --git a/src/cmd/compile/internal/ssa/compile.go b/src/cmd/compile/internal/ssa/compile.go
index c749ea9013..ad581b7114 100644
--- a/src/cmd/compile/internal/ssa/compile.go
+++ b/src/cmd/compile/internal/ssa/compile.go
@@ -476,6 +476,7 @@ var passes = [...]pass{
        {name: "branchelim", fn: branchelim},
        {name: "late opt", fn: opt, required: true}, // TODO: split required rules and optimizing rules
        {name: "dead auto elim", fn: elimDeadAutosGeneric},
+       {name: "generic deadcode for sccp", fn: deadcode},
        {name: "sccp", fn: sccp},
        {name: "generic deadcode", fn: deadcode, required: true}, // remove dead stores, which otherwise mess up store chain
        {name: "late fuse", fn: fuseLate},

By grepping and sorting through sccp's debug info, while building std here is the number of hits:

  1364 before.processed.sorted
   863 after.processed.sorted

before means without the diff above (without rerunning deadcode right before sccp)
after means with the diff above (rerunning deadcode right before sccp)

Very rough timings suggest this is a net compilation speed win.
I'll post correctly measured benchmarks later.

[gabyhelp]

Related Code Changes

• cmd/compile: sparse conditional constant propagation
• [dev.ssa] cmd/compile: use SCC to limit scope of iteration in phi location
• cmd/compile: sparse conditional constant propagation
• cmd/compile: clean up SSA-building code
• cmd/compile: make deadcode pass cheaper
• cmd/compile/internal/ssa: split lowered optimization passes
• cmd/compile/internal/ssa: handle more cases in fuse pass
• cmd/compile: make dead auto elim smarter on indexing
• internal/ssa: implemented CFG decomposition into SCCs to improve live range analysis.
• cmd/compile: deadcode during fuseIf

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