glsl: Store glsl_type::vector_elements and ::matrix_columns as uint8_t

glsl: Store glsl_type::vector_elements and ::matrix_columns as uint8_t

Due to the total number of bits used in the bitfield, this does not
increase the size of the structure.

It does, however, reduce the number of instructions required each time
one of these fields is accessed.  To access ::matrix_columns with the
bitfield, three instructions were required:

    movzbl 0x9(%rdx),%eax
    shr    %al
    and    $0x7,%eax

As a uint8_t, only one instruction is required.

    movzbl 0xa(%rdx),%eax

These fields are accessed *a lot*.

Valgrind callgrind results for a trace of Tesseract:

                 _mesa_Uniform4fv  _mesa_Uniform4f  _mesa_Uniform1i
Before (64-bit):       48,103,497       16,556,096          676,447
After  (64-bit):       45,722,616       15,737,964          670,607

                 _mesa_Uniform4fv  _mesa_Uniform4f  _mesa_Uniform1i
Before (32-bit):       61,472,611       21,051,222          821,361
After  (32-bit):       57,987,421       19,872,226          811,609

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>