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

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

   value -= value

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

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111241

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Questions:

• I'm not sure if this is the correct place to implement this?
• maybe a better fix would be to ask LLVM to not emit NaN in this case, but a more sensible value (0 ?). But maybe this doesn't make sense from the compiler perspective.

v1: implemented for tgsi

v2: implemented in glsl/opt_algebraic.c