This is a follow-up patch to D50166. It modifies frame lowering to emit pseudo instructions corresponding to unwind codes (eg. SEH_SaveReg), that the patch in D50166 (MCLayer part) will emit into the .xdata section. This includes pairing of only consecutive registers in prologue and epilogue, and saving/restoring fp,lr instead of lr,fp on Windows when unwinding data needs to be emitted.
Changes for funclets and EH tables where they differ from X86 will be upstreamed in separate patches.