Move validation to validate.dart and add comments.

sdk/lib/_internal/compiler/implementation/ssa/validate.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/validate.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/validate.dart b/sdk/lib/_internal/compiler/implementation/ssa/validate.dart
index e751b4dcce6c206f6edc371e919fe2656978e10c..5d916194880b1fcb283317545dea560030820c52 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/validate.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/validate.dart
@@ -36,7 +36,16 @@ class HValidator extends HInstructionVisitor {
       markInvalid("If node without two successors");
     }
     if (block.last is HConditionalBranch && block.successors.length != 2) {
-      markInvalid("Conditional node without two successors");
+        markInvalid("Conditional node without two successors");
+      }
+    if (block.last is HLoopBranch) {

[ngeoffray, 2014/06/24 15:45:00]

Why just limiting to HLoopBranch? A generic critical edge detector might be
better.

[floitsch, 2014/06/24 18:29:43]

Done.

+      // Assert that the block we inserted to avoid critical edges satisfies
+      // our assumptions. That is, it must not contain any instructions
+      // (although it may contain phy-updates).

[ngeoffray, 2014/06/24 15:45:01]

phy -> phi

[floitsch, 2014/06/24 18:29:43]

Done.

+      HBasicBlock avoidCriticalEdgeBlock = block.successors.last;
+      if (avoidCriticalEdgeBlock.first is! HGoto) {
+        markInvalid("Critical edge block contains instructions");
+      }
     }
     if (block.last is HGoto && block.successors.length != 1) {
       markInvalid("Goto node with not exactly one successor");