VM: Improve phi-elimination to improve code inside of try-catch.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 3869329698d03e1673511807f032458ec79c6663..db2f1f60da53e4dc0aaa8f1e236626e6c8cc6d8e 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -3520,6 +3520,22 @@ bool PhiInstr::IsRedundant() const {
 }
 
 
+bool PhiInstr::HasReplacement(Definition** replacement) const {
+  ASSERT(InputCount() > 1);
+  ASSERT(!IsRedundant());
+  // At least >=1 inputs are different from 'this': Check for
+  // v <- phi (v, v, ..., r, v, ..., v)
+  // where r is the replacement.
+  intptr_t i = 0;
+  while (InputAt(i)->definition() == this) ++i;
+  *replacement = InputAt(i)->definition();
+  while (++i < InputCount()) {
+    if (InputAt(i)->definition() != this) return false;
+  }
+  return true;
+}
+
+
 bool CheckArrayBoundInstr::IsFixedLengthArrayType(intptr_t cid) {
   return LoadFieldInstr::IsFixedLengthArrayCid(cid);
 }