glsl: replace 'x + (-x)' with constant 0

This fixes a hang in shadertoy for radeonsi where a buffer was initialized with:

   value -= value

with value being undefined.
In this case LLVM replace the operation with an assignment to NaN.

Cc: 19.1 19.2 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111241


Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 47cc660d)