[X86] - Removing warning on legal cases caused by commit r258132

There's an overloading of the "movsd" and "cmpsd" instructions, e.g. movsd can be either "Move Data from String to String" or "Move or Merge Scalar Double-Precision Floating-Point Value".
The former should produce warnings when parsing a memory operand that is not ESI/EDI, but the latter should not.

Fixed the code to produce warnings only after making sure we're dealing with the first case.

Expanded the tests of the produced warnings + fixed RUN line of the test so that it would check both stdout and stderr

Differential Revision: http://reviews.llvm.org/D16359

llvm-svn: 258393