Have AssemblyParse_x86::get_non_call_site_unwind_plan track

which registers have been spilled (saved to the stack) - and
if we see that same register being saved to the stack again,
don't record that, it's something specific to this stack frame.

I found a code sequence for i386 where clang did a push %esi
and then later in the function it did movl %esi, -0x7c(%ebp)
and that second save of a scratch value overrode the original
push location.

<rdar://problem/19171178> 

llvm-svn: 225431