glsl: Fix broken LRP algebraic optimization.

opt_algebraic was translating lrp(x, 0, a) into add(x, -mul(x, a)).

Unfortunately, this references "x" twice, which is invalid in the IR,
leading to assertion failures in the validator.

Normally, cloning IR solves this.  However, "x" could actually be an
arbitrary expression tree, so copying it could result in huge piles
of wasted computation.  This is why we avoid reusing subexpressions.

Instead, transform it into mul(x, add(1.0, -a)), which is equivalent
but doesn't need two references to "x".

Fixes a regression since d5fa8a95621169, which isn't in any stable
branches.  Fixes 18 shaders in shader-db (bastion and yofrankie).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

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