Add a new kind of MachineOperand: MO_TargetIndex.

A target index operand looks a lot like a constant pool reference, but
it is completely target-defined. It contains the 8-bit TargetFlags, a
32-bit index, and a 64-bit offset. It is preserved by all code generator
passes.

TargetIndex operands can be used to carry target-specific information in
cases where immediate operands won't suffice.

llvm-svn: 161441