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

src/IceTargetLoweringARM32.h

Index: src/IceTargetLoweringARM32.h
diff --git a/src/IceTargetLoweringARM32.h b/src/IceTargetLoweringARM32.h
index 4c95d3c0d88fdad283ea493721c4ccd31e06ca09..87faf01567cac99c86b5ec629768adcc7d0854dc 100644
--- a/src/IceTargetLoweringARM32.h
+++ b/src/IceTargetLoweringARM32.h
@@ -103,6 +103,7 @@ public:
   bool hasCPUFeature(TargetARM32Features::ARM32InstructionSet I) const {
     return CPUFeatures.hasFeature(I);
   }
+  Operand *legalizeUndef(Operand *From, int32_t RegNum = Variable::NoRegister);
 
 protected:
   explicit TargetARM32(Cfg *Func);
@@ -146,7 +147,6 @@ 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);
   OperandARM32Mem *formMemoryOperand(Operand *Ptr, Type Ty);
 
   Variable *makeReg(Type Ty, int32_t RegNum = Variable::NoRegister);