This addresses PR 91860 which has four testcases triggering internal errors.
The problem here is that in combine when handling debug insns, we are trying
to substitute
(sign_extend:DI (const_int 8160 [0x1fe0]))
as the value for
(reg:DI 78 [ _9 ])
in the debug insn
(debug_insn 29 28 30 2 (var_location:QI d (subreg:QI (reg:DI 78 [ _9 ]) 0)) "tmp4.c":11:5 -1
     (nil))
This eventually triggers an abort because 8160 is not a sign-extended
QImode value.

In subst there is already code check for a CONST_INT inside a ZERO_EXTEND and
simplify it.  This needs to be extended to also handle a SIGN_EXTEND the same
way.

	gcc/
	PR rtl-optimization/91860
	* combine.c (subst): If new_rtx is a constant, also check for
	SIGN_EXTEND when deciding whether to call simplify_unary_operation.

	gcc/testsuite/
	PR rtl-optimization/91860
	* gcc.dg/pr91860-1.c: New testcase.
	* gcc.dg/pr91860-2.c: New testcase.
	* gcc.dg/pr91860-3.c: New testcase.
	* gcc.dg/pr91860-4.c: New testcase.

From-SVN: r276901