cse: Fix up CSE const_anchor handling [PR108193]

cse: Fix up CSE const_anchor handling [PR108193]

The following testcase ICEs on aarch64, because insert_const_anchor
inserts invalid CONST_INT into the CSE tables - 0x80000000 for SImode.
The second hunk of the patch fixes that, the first one is to avoid
triggering undefined behavior at compile time during compute_const_anchors
computations - performing those additions and subtractions in
HOST_WIDE_INT means it can overflow for certain constants.

2022-12-22  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/108193
	* cse.cc (compute_const_anchors): Change n type to
	unsigned HOST_WIDE_INT, adjust comparison against it to avoid
	warnings.  Formatting fix.
	(insert_const_anchor): Use gen_int_mode instead of GEN_INT.

	* gfortran.dg/pr108193.f90: New test.