[MC] [Win64EH] Try writing an ARM64 "packed epilog" even if the epilog doesn't share...

[MC] [Win64EH] Try writing an ARM64 "packed epilog" even if the epilog doesn't share opcodes with the prolog

The "packed epilog" form only implies that the epilog is located
exactly at the end of the function (so the location of the epilog
is implicit from the epilog opcodes), but it doesn't have to share
opcodes with the prolog - as long as the total number of opcode
bytes and the offset to the epilog fit within the bitfields.

This avoids writing a 4 byte epilog scope in many cases. (I haven't
measured how much this shrinks actual xdata sections in practice
though.)

Differential Revision: https://reviews.llvm.org/D125536