Cache common constants before lowering.

src/IceTargetLoweringMIPS32.cpp

Index: src/IceTargetLoweringMIPS32.cpp
diff --git a/src/IceTargetLoweringMIPS32.cpp b/src/IceTargetLoweringMIPS32.cpp
index 48b5b0316d8710706daf9f935a95d02ed339ad23..282d6eaeea8d9849a9360203639cd75c675c8d9d 100644
--- a/src/IceTargetLoweringMIPS32.cpp
+++ b/src/IceTargetLoweringMIPS32.cpp
@@ -351,7 +351,7 @@ Operand *TargetMIPS32::legalizeUndef(Operand *From, RegNumT RegNum) {
     // need to go in uninitialized registers.
     if (isVectorType(Ty))
       UnimplementedError(Func->getContext()->getFlags());
-    return Ctx->getConstantZero(Ty);
+    return getConstantZero(Ty);
   }
   return From;
 }