Factor out legalization of undef, and handle more cases for ARM.

src/IceTargetLoweringX86Base.h

Index: src/IceTargetLoweringX86Base.h
diff --git a/src/IceTargetLoweringX86Base.h b/src/IceTargetLoweringX86Base.h
index c00f0b0e5524207dec9ccf1275de84f6ced074e4..69b88ee9d189d15d1e7e2e61b9ff07961154b2a0 100644
--- a/src/IceTargetLoweringX86Base.h
+++ b/src/IceTargetLoweringX86Base.h
@@ -230,6 +230,7 @@ protected:
   Operand *legalize(Operand *From, LegalMask Allowed = Legal_All,
                     int32_t RegNum = Variable::NoRegister);
   Variable *legalizeToVar(Operand *From, int32_t RegNum = Variable::NoRegister);
+  Operand *legalizeUndef(Operand *From, int32_t RegNum = Variable::NoRegister);
   /// Legalize the first source operand for use in the cmp instruction.
   Operand *legalizeSrc0ForCmp(Operand *Src0, Operand *Src1);
   /// Turn a pointer operand into a memory operand that can be