glsl: Don't allow gl_PerVertex to be redeclared after it's been used.

glsl: Don't allow gl_PerVertex to be redeclared after it's been used.

Fixes piglit tests:
- spec/glsl-1.50/compiler/gs-redeclares-pervertex-in-after-other-usage.geom
- spec/glsl-1.50/compiler/gs-redeclares-pervertex-out-after-other-usage.geom
- spec/glsl-1.50/compiler/gs-redeclares-pervertex-out-after-usage.geom
- spec/glsl-1.50/compiler/vs-redeclares-pervertex-out-after-other-usage.vert
- spec/glsl-1.50/compiler/vs-redeclares-pervertex-out-after-usage.vert

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>