ifcvt: Don't lower bitfields with non-constant offsets [PR 111882]

This patch stops lowering of bitfields by ifcvt when they have non-constant
offsets as we are not likely to be able to do anything useful with those during
vectorization.  That also fixes the issue reported in PR 111882, which was
being caused by an offset with a side-effect being lowered, but constants have
no side-effects so we will no longer run into that problem.

gcc/ChangeLog:

	PR tree-optimization/111882
	* tree-if-conv.cc (get_bitfield_rep): Return NULL_TREE for bitfields
	with non-constant offsets.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/pr111882.c: New test.