glsl: Add simple vector type accessor helpers.

glsl: Add simple vector type accessor helpers.

This patch introduces new functions to quickly grab a pointer to a
vector type.  For example:

   glsl_type::bvec(4)   returns   glsl_type::bvec4_type
   glsl_type::ivec(3)   returns   glsl_type::ivec3_type
   glsl_type::uvec(2)   returns   glsl_type::uvec2_type
   glsl_type::vec(1)    returns   glsl_type::float_type

This is less wordy than glsl_type::get_instance(GLSL_TYPE_BOOL, 4, 1),
which can help avoid extra word wrapping.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>