glsl: do not attempt assignment if operand type not parsed correctly

v2: check types of both operands (Ian)

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108012
(cherry picked from commit d1fa69ed)