Use Is*() helpers to shorten code in the optimizing compiler.

Change-Id: I79f31833bc9a0aa2918381aa3fb0b05d45f75689
diff --git a/compiler/optimizing/ssa_builder.cc b/compiler/optimizing/ssa_builder.cc
index 471307e..be2c039 100644
--- a/compiler/optimizing/ssa_builder.cc
+++ b/compiler/optimizing/ssa_builder.cc
@@ -51,7 +51,7 @@
        !it.Done();
        it.Advance()) {
     HInstruction* current = it.Current();
-    if (current->AsLocal() != nullptr) {
+    if (current->IsLocal()) {
       current->GetBlock()->RemoveInstruction(current);
     }
   }