glsl: skip comparison opt when adding vars of different size

The spec allows adding scalars with a vector or matrix. In this case
the opt was losing swizzle and size information.

This fixes a bug with Doom (2016) shaders.

Fixes: 34ec1a24 ("glsl: Optimize (x + y cmp 0) into (x cmp -y).")

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 2a5121bf)