[RS6000] PR70052, ICE compiling _Decimal128 test case

[RS6000] PR70052, ICE compiling _Decimal128 test case

gcc/
	PR target/70052
	* config/rs6000/constraints.md (j): Simplify.
	* config/rs6000/predicates.md (easy_fp_constant): Exclude
	decimal float 0.D.
	* config/rs6000/rs6000.md (zero_fp): New mode_attr.
	(mov<mode>_hardfloat, mov<mode>_hardfloat32, mov<mode>_hardfloat64,
	 mov<mode>_64bit_dm, mov<mode>_32bit): Use zero_fp in place of j
	in all constraint alternatives.
	(movtd_64bit_nodm): Delete "j" constraint alternative.
gcc/testsuite/
	* gcc.dg/dfp/pr70052.c: New test.

From-SVN: r234479