glsl: Eliminate out-of-bounds writes to gl_TessLevel*
Out-of-bounds writes could be eliminated per spec:
Section 5.11 (Out-of-Bounds Accesses) of the GLSL 4.60 spec says:
"In the subsections described above for array, vector, matrix and
structure accesses, any out-of-bounds access produced undefined
behavior.... Out-of-bounds writes may be discarded or overwrite
other variables of the active program."
Fixes: 73a9a153
Fixes tcs-tes-levels-out-of-bounds-write.shader_test
from piglit!373 (merged) on at least llvmpipe.