[AArch64] Exclude instructions after setting the FP from SEH prologues

After setting up the FP, the rest of the prologue doesn't need to
be replayed for unwinding the stack frame.

This allows reverting the functional parts of
2f7fbf83 (but fixing inconsistent
duplicate setting of HasWinCFI).

Differential Revision: https://reviews.llvm.org/D135686