i965: Avoid explicit accumulator operands in SIMD16 mode on Gen7.

According to the BSpec ISA volume's "Accumulator Register" section:

"[DevIVB] SIMD16 execution on dwords is not allowed when accumulator is
 explicit source or destination operand."

Fixes piglit tests:
- fs-multiply-const-ivec4
- fs-multiply-const-uvec4
- fs-multiply-ivec4-const
- fs-multiply-uvec4-const

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs_visitor.cpp

@@ -337,6 +337,9 @@ fs_visitor::visit(ir_expression *ir)
 	  * FINISHME: Emit just the MUL if we know an operand is small
 	  * enough.
 	  */
+	 if (intel->gen >= 7 && c->dispatch_width == 16)
+	    fail("16-wide explicit accumulator operands unsupported\n");
+
 	 struct brw_reg acc = retype(brw_acc_reg(), BRW_REGISTER_TYPE_D);
 
 	 emit(BRW_OPCODE_MUL, acc, op[0], op[1]);