glsl: Convert TES gl_PatchVerticesIn into a constant when using a TCS.

glsl: Convert TES gl_PatchVerticesIn into a constant when using a TCS.

When a TCS is present, the TES input gl_PatchVerticesIn is actually a
constant - it's simply the # of output vertices specified by the TCS
layout qualifiers.  So, we can replace the system value with a constant,
which may allow further optimization, and will likely be more efficient.

If the TCS is absent, we can't do this optimization.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>