[MC] [Win64EH] Check that ARM64 prologs and epilogs have the right matching number of instructions

This matches what was done for the ARM implementation (where getting
the instruction sizes right is even more tricky, and hence needed
tighter testing).

This will allow catching any future cases where prologs and epilogs
don't match the instructions within them.

Differential Revision: https://reviews.llvm.org/D131394