[x86] Rewrite getAddressFromInstr helper function

- It does not modify the input instruction
- Second operand of any address is always an Index Register,
  make sure we actually check for that, instead of a check for
  an immediate value

Patch by Alexander Ivchenko <alexander.ivchenko@intel.com>

Differential Revision: https://reviews.llvm.org/D24938

llvm-svn: 287873