Fixing bug in rL258132: [X86] Adding support for missing variations of X86...

Fixing bug in rL258132: [X86] Adding support for missing variations of X86 string related instructions

There was a bug in my rL258132 because there's an overloading of the "movsd" and "cmpsd" instructions, e.g. movsd can be either "Move Data from String to String" (the case I wanted to handle) or "Move or Merge Scalar Double-Precision Floating-Point Value" (the case that causes the asserts).
Added  code for escaping the unfamiliar scenarios and falling back to old behviour.
Also changed the asserts to llvm_unreachable. 

llvm-svn: 258312