glsl: Lower ir_binop_vector_extract to conditional moves

glsl: Lower ir_binop_vector_extract to conditional moves

Lower ir_binop_vector_extract with a non-constant index to a series of
conditional moves.  This is exactly like ir_dereference_array of a
vector with a non-constant index.

v2: Convert tabs to spaces.  Suggested by Eric.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>