glsl: Drop no-op shifts involving 0.

I noticed this in a shader in Unigine Heaven that was spilling.  While it
doesn't really reduce register pressure, it shaves a few instructions
anyway (7955 -> 7882).

v2: Fix turning "0 >> x" into "x" instead of "0" (caught by Erik
    Faye-Lund).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/glsl/opt_algebraic.cpp

@@ -346,6 +346,16 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
       }
       break;
 
+   case ir_binop_rshift:
+   case ir_binop_lshift:
+      /* 0 >> x == 0 */
+      if (is_vec_zero(op_const[0]))
+         return ir->operands[0];
+      /* x >> 0 == x */
+      if (is_vec_zero(op_const[1]))
+         return ir->operands[0];
+      break;
+
    case ir_binop_logic_and:
       /* FINISHME: Also simplify (a && a) to (a). */
       if (is_vec_one(op_const[0])) {