ir_constant: Return zero on out-of-bounds vector accesses

ir_constant: Return zero on out-of-bounds vector accesses

Several optimization paths, including constant folding, can lead to
accessing an ir_constant vector with an out of bounds index.

Return 0 since GL_ARB_robustness and GL_KHR_robustness encourage
us to do so.

Fixes piglit tests:
spec@glsl-1.20@execution@vector-out-of-bounds-access@fs-vec4-out-of-bounds-2
spec@glsl-1.20@execution@vector-out-of-bounds-access@fs-vec4-out-of-bounds-4
spec@glsl-1.20@execution@vector-out-of-bounds-access@fs-vec4-out-of-bounds-5

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2604
CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6363>