PR 106101: IBM zSystems: Fix strict_low_part problem

PR 106101: IBM zSystems: Fix strict_low_part problem

This avoids generating illegal (strict_low_part (reg ...)) RTXs. This
required two changes:

1. Do not use gen_lowpart to generate the inner expression of a
STRICT_LOW_PART.  gen_lowpart might fold the SUBREG either because
there is already a paradoxical subreg or because it can directly be
applied to the register. A new wrapper function makes sure that we
always end up having an actual SUBREG.

2. Change the movstrict patterns to enforce a SUBREG as inner operand
of the STRICT_LOW_PARTs.  The new predicate introduced for the
destination operand requires a SUBREG expression with a
register_operand as inner operand.  However, since reload strips away
the majority of the SUBREGs we have to accept single registers as well
once we reach reload.

Bootstrapped and regression tested on IBM zSystems 64 bit.

gcc/ChangeLog:

	PR target/106101
	* config/s390/predicates.md (subreg_register_operand): New
	predicate.
	* config/s390/s390-protos.h (s390_gen_lowpart_subreg): New
	function prototype.
	* config/s390/s390.cc (s390_gen_lowpart_subreg): New function.
	(s390_expand_insv): Use s390_gen_lowpart_subreg instead of
	gen_lowpart.
	* config/s390/s390.md ("*get_tp_64", "*zero_extendhisi2_31")
	("*zero_extendqisi2_31", "*zero_extendqihi2_31"): Likewise.
	("movstrictqi", "movstricthi", "movstrictsi"): Use the
	subreg_register_operand predicate instead of register_operand.

gcc/testsuite/ChangeLog:

	PR target/106101
	* gcc.c-torture/compile/pr106101.c: New test.

(cherry picked from commit 585a21bab3ec688c2039bff2922cc372d8558283)