i965: add a new virtual opcode: SHADER_OPCODE_TXF_MS

i965: add a new virtual opcode: SHADER_OPCODE_TXF_MS

This is very similar to the TXF opcode, but lowers to `ld2dms` rather
than `ld` on Gen7.

V4: - add SHADER_OPCODE_TXF_MS to is_tex() functions, so regalloc thinks
      it actually writes the correct number of registers. Otherwise in
      nontrivial shaders some of the registers tend to get clobbered,
      producing bad results.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>