- Detect code paths in non-void functions that don't reach a return statement

- Improve handling of constants and their initializers.  Constant initializers
  should never generate any code.  This is trival for scalar constants.  It is
  also trivial for arrays, matrices, and vectors that are accessed with
  constant index values.  For others it is more complicated.  Perhaps these
  cases should be silently converted to uniforms?

- Implement support for ir_binop_dot in ir_algebraic.cpp.  Perform
  transformations such as "dot(v, vec3(0.0, 1.0, 0.0))" -> v.y.

1.30 features:

- Implement AST-to-HIR conversion of bit-shift operators.

- Implement AST-to-HIR conversion of bit-wise {&,|,^,!} operators.

- Implement AST-to-HIR conversion of switch-statements
  - switch
  - case
  - Update break to correcly handle mixed nexting of switch-statements
    and loops.

- Handle currently unsupported constant expression types
  - ir_unop_bit_not
  - ir_binop_mod
  - ir_binop_lshift
  - ir_binop_rshift
  - ir_binop_bit_and
  - ir_binop_bit_xor
  - ir_binop_bit_or

- Implement support for 1.30 style shadow compares which only return a float
  instead of a vec4.

- Implement support for gl_ClipDistance.  This is non-trivial because
  gl_ClipDistance is exposed as a float[8], but all hardware actually
  implements it as vec4[2].