- 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?

- Track source locations throughout the IR.  There are currently several
  places where we cannot emit line numbers for errors (and currently emit 0:0)
  because we've "lost" the line number information.  This is particularly
  noticeable at link time.