fold undefined pointer offsetting

fold undefined pointer offsetting

This avoids breaking the old broken pointer offsetting via
(T)(ptr - ((T)0)->x) which should have used offsetof.  Breakage
was exposed by the introduction of POINTER_DIFF_EXPR and making
PTA not considering that producing a pointer.  The mitigation
for simple cases is to canonicalize

  _2 = _1 - 8B;
  o_9 = (struct obj *) _2;

to

  o_9 = &MEM[_1 + -8B];

eliding one statement and the offending pointer subtraction.

2020-03-11  Richard Biener  <rguenther@suse.de>

	* match.pd ((T *)(ptr - ptr-cst) -> &MEM[ptr + -ptr-cst]):
	New pattern.

	* gcc.dg/torture/20200311-1.c: New testcase.