glsl: Emit a warning when the left-hand operand of a comma has no effect

glsl: Emit a warning when the left-hand operand of a comma has no effect

The expression

    x = y, 5, 3;

will generate

    0:7(9): warning: left-hand operand of comma expression has no effect

The warning is only emitted for the left-hand operands, becuase the
right-most operand is the result of the expression.  This could be
used in an assignment, etc.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>