glsl/opt_cpe: Kill when the assignment isn't something we recognize.
A few GLES2 tests tripped over this when using array dereferences to hit channels on the LHS (see piglit test glsl-copy-propagation-vector-indexing). We wouldn't find the ir_dereference_variable, and assume that that meant that it wasn't an assignment to a scalar/vector, and thus not notice that the variable had been changed.