Factor out prelowerPhi for 32-bit targets. Disable adv phi lowering for ARM.

src/IceTargetLoweringX86Base.h

Index: src/IceTargetLoweringX86Base.h
diff --git a/src/IceTargetLoweringX86Base.h b/src/IceTargetLoweringX86Base.h
index 69b88ee9d189d15d1e7e2e61b9ff07961154b2a0..7963b01bdfdd2c2d03f187f587a831204d432fb4 100644
--- a/src/IceTargetLoweringX86Base.h
+++ b/src/IceTargetLoweringX86Base.h
@@ -159,6 +159,7 @@ public:
   typename Traits::InstructionSet getInstructionSet() const final {
     return InstructionSet;
   }
+  Operand *legalizeUndef(Operand *From, int32_t RegNum = Variable::NoRegister);
 
 protected:
   explicit TargetX86Base(Cfg *Func);
@@ -230,7 +231,6 @@ 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