glsl: set error_emitted true if type not ok for assignment

glsl: set error_emitted true if type not ok for assignment

Patch changes also existing assert to not trigger when we have
error types in assignment.

v2: simplify, cleanup (Ian)

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2629
Fixes: d1fa69ed61d ("glsl: do not attempt assignment if operand type not parsed correctly")
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4178>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4178>