2018-02-16  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/84417
	* tree-ssa.c (non_rewritable_mem_ref_base): Properly constrain
	the MEM_REF offset when conversion to BIT_FIELD_REF is desired.
	(non_rewritable_lvalue_p): Likewise, use poly-ints.

	* gcc.dg/torture/pr84417.c: New testcase.

From-SVN: r257731