glsl: Fix type mismatch when incrementing or decrementing uint.

glsl: Fix type mismatch when incrementing or decrementing uint.

When converting an expression like "++x" to GLSL IR we were failing to
account for the possibility that x might be an unsigned integral type.
As a result the user would receive a bogus error message "Could not
implicitly convert operands to arithmetic operator".

Fixes piglit tests {vs,fs}-{increment,decrement}-uint.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>