re PR fortran/30381 ([4.1 only] ISHFTC() constant folding is broken.)

re PR fortran/30381 ([4.1 only] ISHFTC() constant folding is broken.)

PR 30381
PR 30420
* fortran/simplify.c (convert_mpz_to_unsigned): New function.
	(convert_mpz_to_signed): New function, largely based on
	twos_complement().
	(twos_complement): Removed.
	(gfc_simplify_ibclr): Add conversions to and from an
	unsigned representation before bit-twiddling.
	(gfc_simplify_ibset): Same.
	(gfc_simplify_ishftc): Add checks for overly large
	constant arguments, only check the third argument if
	it's present, carry over high bits into	the result as
	appropriate, and perform the final conversion back to
	a signed representation using the correct sign bit.
	(gfc_simplify_not): Removed unnecessary masking.
* testsuite/gfortran.dg/
	* chkbits.f90: Added IBCLR tests; test calls for
	different integer kinds.
	* ishft.f90: Renamed to ishft_1.f90...
	* ishft_1.f90: ...Renamed from ishft.f90.
	* ishft_2.f90: New test.
	* ishft_3.f90: New test.

From-SVN: r120634