Revert a fix for constrained polymorphic modes.

Commit 338b5cb98e8abd320aac6d3619f8ab98f773c846 changed a call in
handle_inst_var_subs_2, essentially from
	Recurse(InstA, SubInstB)
to
	Recurse(InstA, abstractly_unify_inst(InstA, SubInstB))

Unfortunately, it results in infinite recursion on some cases.

compiler/inst_match.m:
	Revert the change to handle_inst_var_subs_2.

tests/EXPECT_FAIL_TESTS.all_grades:
	Expect constrained_poly_insts2 to fail now.

tests/invalid/constrained_poly_insts3.err_exp:
tests/invalid/constrained_poly_insts3.m:
tests/invalid/constrained_poly_insts4.err_exp:
tests/invalid/constrained_poly_insts4.m:
	Add test cases.