Stack unwinding emulation: handle adjustment of FP

This change is improving the instruction emulation based unwinding to
handle when the frame pointer is adjusted (increment/decrement) after
it has been initialized. The situation can occur in the prologue of
some function where FP is adjusted before it is copied back to SP.

Example code (thumb, generated by gcc 4.8):
< +0>: push  {r4, r7, lr}
< +2>: sub   sp, #0x14
< +4>: add   r7, sp, #0x0
...
<+50>: adds  r7, #0x14 ; The CL fixes the handling of this instruction
<+52>: mov   sp, r7    ; Previously unwinding from here was broken
<+54>: pop   {r4, r7, pc}

Differential revision: http://reviews.llvm.org/D17295

llvm-svn: 261318