nir/types: Make glsl_get_length smarter

nir/types: Make glsl_get_length smarter

Previously, this function returned the number of elements for structures
and arrays and 0 for everything else.  In NIR, this is almost never what
you want because we also treat matricies as arrays so you have to
special-case constantly.  This commit  glsl_get_length treat matrices
as an array of columns by returning the number of columns instead of 0

This also fixes a bug in locals_to_regs caused by not checking for the
matrix case in one place.

v2: Only special-case for matrices and return a length of 0 for vectors as
    we did before.  This was needed to not break the TGSI-based drivers and
    doesn't really affect NIR at the moment.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Tested-by: Rob Clark <robclark@freedesktop.org>