Allow for testing alternative code generators in codegen_test.cc

This will be used in a later patch to test a new VIXL32-based backend
in parallel with the existing code_generator_arm.

Test: gtest-codegen_test on host and target

Change-Id: I0316da0430fa6da0a7c668315f531888d18e7eb3
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 19e499b..61f4572 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -109,6 +109,9 @@
   kCondBE,  // <=
   kCondA,   // >
   kCondAE,  // >=
+  // First and last aliases.
+  kCondFirst = kCondEQ,
+  kCondLast = kCondAE,
 };
 
 enum GraphAnalysisResult {