glsl: add support for ARB_texture_multisample

V2: - emit `sample` parameter properly for multisample texelFetch()
    - fix spurious whitespace change
    - introduce a new opcode ir_txf_ms rather than overloading the
      existing ir_txf further. This makes doing the right thing in
      the driver somewhat simpler.

V3: - fix weird whitespace

V4: - don't forget to include the new opcode in tex_opcode_strs[]
      (thanks Kenneth for spotting this)

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
[V2] Reviewed-by: Eric Anholt <eric@anholt.net>
[V2] Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ir_rvalue_visitor.cpp

@@ -66,6 +66,9 @@ ir_rvalue_base_visitor::rvalue_visit(ir_texture *ir)
    case ir_txs:
       handle_rvalue(&ir->lod_info.lod);
       break;
+   case ir_txf_ms:
+      handle_rvalue(&ir->lod_info.sample_index);
+      break;
    case ir_txd:
       handle_rvalue(&ir->lod_info.grad.dPdx);
       handle_rvalue(&ir->lod_info.grad.dPdy);