glsl: Use vector-insert and vector-extract on elements of gl_ClipDistanceMESA

glsl: Use vector-insert and vector-extract on elements of gl_ClipDistanceMESA

Variable indexing into vectors using ir_dereference_array is being
removed, so this lowering pass has to generate something different.

v2: Convert tabs to spaces.  Suggested by Eric.

v3: Simplify code slightly by assuming that elements of
gl_ClipDistanceMESA will always be vec4.  Suggested by Paul.

v4: Fairly substantial rewrite based on the rewrite of "glsl: Convert
lower_clip_distance_visitor to be an ir_rvalue_visitor"

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>