ubsan: Fix ICEs with DECL_REGISTER tests [PR101624]

ubsan: Fix ICEs with DECL_REGISTER tests [PR101624]

The following testcase ICEs, because the base is a CONST_DECL for
the Fortran parameter, and ubsan/sanopt uses DECL_REGISTER macro on it.
 /* In VAR_DECL and PARM_DECL nodes, nonzero means declared `register'.  */
 #define DECL_REGISTER(NODE) (DECL_WRTL_CHECK (NODE)->decl_common.decl_flag_0)
while CONST_DECL doesn't satisfy DECL_WRTL_CHECK.

The following patch checks explicitly for VAR_DECL/PARM_DECL/RESULT_DECL
only before using DECL_REGISTER, assumes other decls aren't DECL_REGISTER.
Not really sure about RESULT_DECL but it at least satisfies DECL_WRTL_CHECK...

2021-07-28  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101624
	* ubsan.c (maybe_instrument_pointer_overflow,
	instrument_object_size): Only test DECL_REGISTER on VAR_DECLs,
	PARM_DECLs or RESULT_DECLs.
	* sanopt.c (maybe_optimize_ubsan_ptr_ifn): Likewise.

	* gfortran.dg/ubsan/ubsan.exp: New file.
	* gfortran.dg/ubsan/pr101624.f90: New test.