c: Handle MEM_REF in c_fully_fold* [PR94179]

c: Handle MEM_REF in c_fully_fold* [PR94179]

The recent match.pd changes can generate a MEM_REF which can be seen by the
C FE folding routines.  Unlike the C++ FE, they weren't expected in the C FE
yet.  MEM_REF should be handled like INDIRECT_REF, except that it has two
operands rather than just one and that we should preserve the type of the
second operand.  Given that it already has to be an INTEGER_CST with pointer
type, I think we are fine, the recursive call should return the INTEGER_CST
unmodified and STRIP_TYPE_NOPS will not strip anything.

2020-03-16  Jakub Jelinek  <jakub@redhat.com>

	PR c/94179
	* c-fold.c (c_fully_fold_internal): Handle MEM_REF.

	* gcc.c-torture/compile/pr94179.c: New test.