[ARM] Rename the isARMAreaXRegister parameter isIOS to SplitFramePushPop. NFC.

[mstorsjo]

Summary

In f8b0a7af52f8c4ec6b4ddcfe3a6fa75098c9507c in 2016, this parameter
was generalized on the caller side (previously passing
STI.isTargetMachO(), now passing STI.splitFramePushPop()). Rename
the parameter on the receiver side to match the generalization.

[DavidSpickett]

LGTM

llvm/lib/Target/ARM/ARMBaseRegisterInfo.h

enum {
     // Used to hint for lr in t2DoLoopStart
     RegLR = 3
   };
 
 } // end namespace ARMRI
 
 /// isARMArea1Register - Returns true if the register is a low register (r0-r7)
 /// or a stack/pc register that we should push/pop.
-static inline bool isARMArea1Register(unsigned Reg, bool isIOS) {
+static inline bool isARMArea1Register(unsigned Reg, bool SplitFramePushPop) {
   using namespace ARM;
 
   switch (Reg) {
     case R0:  case R1:  case R2:  case R3:
     case R4:  case R5:  case R6:  case R7:
     case LR:  case SP:  case PC:
       return true;
     case R8:  case R9:  case R10: case R11: case R12:
       // For iOS we want r7 and lr to be next to each other.
-      return !isIOS;
+      return !SplitFramePushPop;
     default:
       return false;
   }
 }
 
-static inline bool isARMArea2Register(unsigned Reg, bool isIOS) {
+static inline bool isARMArea2Register(unsigned Reg, bool SplitFramePushPop) {
   using namespace ARM;
 
   switch (Reg) {
     case R8: case R9: case R10: case R11: case R12:
       // iOS has this second area.
-      return isIOS;
+      return SplitFramePushPop;
     default:
       return false;
   }
 }
 
-static inline bool isARMArea3Register(unsigned Reg, bool isIOS) {
+static inline bool isARMArea3Register(unsigned Reg, bool SplitFramePushPop) {
   using namespace ARM;
 
   switch (Reg) {
     case D15: case D14: case D13: case D12:
     case D11: case D10: case D9:  case D8:
     case D7:  case D6:  case D5:  case D4:
     case D3:  case D2:  case D1:  case D0:
     case D31: case D30: case D29: case D28: