Added support for X86 instruction prefixes so llvm-mc can assemble them. The

Lock prefix, Repeat string operation prefixes and the Segment override prefixes.
Also added versions of the move string and store string instructions without the
repeat prefixes to X86InstrInfo.td. And finally marked the rep versions of
move/store string records in X86InstrInfo.td as isCodeGenOnly = 1 so tblgen is
happy building the disassembler files.

llvm-svn: 95252