cmd/compile: adapt GOSSAHASH to set a DebugTest flag in ssa.Config

Binary search remains our friend.
Suppose you add an ought-to-be-benign pattern to PPC64.rules,
and make.bash starts crashing.  You can guard the pattern(s)
with config.DebugTest:

(Eq8 x y) && config.DebugTest && isSigned(x.Type) &&
   isSigned(y.Type) ->
   (Equal (CMPW (SignExt8to32 x) (SignExt8to32 y)))

and then

  gossahash -s ./make.bash
  ...
  (go drink beer while silicon minions toil)
  ...
  Trying ./make.bash args=[], env=[GOSSAHASH=100110010111110]
  ./make.bash failed (1 distinct triggers): exit status 1
  Trigger string is 'GOSSAHASH triggered (*importReader).readByte',
    repeated 1 times
  Review GSHS_LAST_FAIL.0.log for failing run
  Finished with GOSSAHASH=100110010111110

Change-Id: I4eff46ebaf496baa2acedd32e217005cb3ac1c62
Reviewed-on: https://go-review.googlesource.com/29273
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -33,6 +33,7 @@ type Config struct {
 	nacl            bool                       // GOOS=nacl
 	use387          bool                       // GO386=387
 	NeedsFpScratch  bool                       // No direct move between GP and FP register sets
+	DebugTest       bool                       // as a debugging aid for binary search using GOSSAHASH, make buggy new code conditional on this
 	sparsePhiCutoff uint64                     // Sparse phi location algorithm used above this #blocks*#variables score
 	curFunc         *Func