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

In f8b0a7af 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.

Differential Revision: https://reviews.llvm.org/D125681