glsl-1.10: Avoid NIR optimization in gl_FrontFacing test

Use some non-obvious arithmetic to try to avoid some NIR optimizations
that hide a bug in the Intel compiler backend for Gfx12+ (Tigerlake and
newer) GPUs.  Basically, when NIR sees

   x = (gl_FrontFacing ? -1.0 : 1.0) * a;

it converts it to

    x = gl_FrontFacing ? -a : a;

This avoids the bad code generation for the (gl_FrontFacing ? -1.0 :
1.0) case.

v2: Make this a separate test.  Suggested by Marcin.

Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <!627>
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