Add a test case for r242191 ([MMX] Use the appropriate instructions for

GR64 <-> VR64 copies).

This commit adds a MIR test case for the commit r242191, which was committed
without one. This test case verifies that the ExpandPostRA pass expands the
GR64 <-> VR64 copies into the appropriate MMX_MOV instructions.

llvm-svn: 243457