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
2f7fbf837625267193351cc334e506a3a9161958 (but fixing inconsistent
duplicate setting of HasWinCFI).
Differential Revision: https://reviews.llvm.org/D135686
This makes sure that the instructions of the prologue matches the
SEH opcodes.
Also remove a couple redundant cases of setting HasWinCFI; it was
already set unconditionally after the conditional cases.
Differential Revision: https://reviews.llvm.org/D135101