tree-optimization/103759: Use sizetype everywhere for object sizes

Since all computations in tree-object-size are now done in sizetype and
not HOST_WIDE_INT, comparisons with HOST_WIDE_INT based unknown and
initval would be incorrect.  Instead, use the sizetype trees directly to
generate and evaluate initval and unknown size values.

gcc/ChangeLog:

	PR tree-optimization/103759
	* tree-object-size.c (unknown, initval): Remove functions.
	(size_unknown, size_initval, size_unknown_p): Operate directly
	on trees.

Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>