Change how the UnwindAssemblyInstEmulation class tracks the setup of

the function's prologue instructions so we can re-instate that prologue
if we hit an early return mid-function.  Add some additional heuristics
to differentiate between prologue and epilogue instruction sequences.

This fixes the specific problem of correctly unwinding through a function
which has an epilogue one instruction after the last prologue setup
instruction has completed.
<rdar://problem/12091139> 

llvm-svn: 166465