glsl: Implement ast-to-hir for binary shifts in GLSL 1.30

glsl: Implement ast-to-hir for binary shifts in GLSL 1.30

Implement by adding the following cases to ast_expression::hir():
    - ast_lshift
    - ast_rshift
Also, implement ir validation for the new operators by adding the following
cases to ir_validate::visit_leave():
    - ir_binop_lshift
    - ir_binop_rshift