tree-optimization/126650 - get_inner_reference_aff and bitfields

get_inner_reference_aff tries to compensate for bit precision
offsets and sizes by rounding down and up, but fails to do this
correctly.  The following makes it more obviously correct by
rounding positions and computing the rounded size based on those.

	PR tree-optimization/126650
	* tree-affine.cc (get_inner_reference_aff): Fixup rounding
	of size.

	* gcc.dg/torture/pr126650.c: New testcase.

(cherry picked from commit 9baaca2adeca178edf8c0816ef328dba1cf270bc)