StructurizeCFG: fix inverting conditions

Author: Samuel Pitoiset

Without this patch, it appears to me that we are selecting
the wrong operand when inverting conditions. In the attached
test, it will select %tmp3 instead of %tmp4. To fix it, just
use 'A' as everywhere.

This fixes a regression introduced by
"[PatternMatch] define m_Not using m_Xor and cst_pred_ty"

https://reviews.llvm.org/D46351

llvm-svn: 332403