tree.h (sizetype_tab[2], [...]): Merge all of these into a single struct, with additional [us]sizetype entries.

* tree.h (sizetype_tab[2], sbitsizetype, ubitsizetype): Merge all
of these into a single struct, with additional [us]sizetype entries.
* stor-layout.c (set_sizetype): Initialize [us]sizetype.
* fold-const.c (size_int_wide): Don't rely on sizetype_tab being
an array.

From-SVN: r18994