lower-bitint: Handle INTEGER_CST rhs1 in handle_cast [PR113462]

lower-bitint: Handle INTEGER_CST rhs1 in handle_cast [PR113462]

The following patch ICEs because fre3 leaves around unfolded
  _1 = VIEW_CONVERT_EXPR<_BitInt(129)>(0);
statement and in handle_cast I was expecting just SSA_NAMEs for the
large/huge _BitInt to large/huge _BitInt casts; INTEGER_CST is something
we can handle in that case exactly the same, as the handle_operand recursion
handles those.

Of course, maybe we should also try to fold_stmt such cases somewhere in
bitint lowering preparation.

2024-01-20  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113462
	* gimple-lower-bitint.cc (bitint_large_huge::handle_cast):
	Handle rhs1 INTEGER_CST like SSA_NAME.

	* gcc.dg/bitint-76.c: New test.