i965: Make brw_type_for_base_type return the element type for arrays.

i965: Make brw_type_for_base_type return the element type for arrays.

Previously, brw_type_for_base_type returned UD for array variables,
similar to structures.  For structures, each field may have a different
type, so every field access must explicitly override the register's type
with that field's type.  We chose to return UD in this case since it was
the least common, so errors would be more obvious.

For arrays, it makes far more sense to return the type corresponding to
an element of the array.  This allows normal array access to work
without the hassle of explicitly overriding the register's type.

This should obsolete a bunch of type overrides throughout the code.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>