glsl: Generate code for constant ir_binop_lshift and ir_binop_rshift expressions

glsl: Generate code for constant ir_binop_lshift and ir_binop_rshift expressions

The code generated is quite different from what was previously used.  I
believe that it is still correct by the GLSL spec, and I believe, due to
C rules about shifts, the behavior will be the same.

Section 5.9 (Expressions) of the GLSL 4.50 spec says:

    The result is undefined if the right operand is negative, or greater
    than or equal to the number of bits in the left expression's base
    type.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>