s390: Fix constraint for insn *cmphi_ccu

s390: Fix constraint for insn *cmphi_ccu

Currently for an unsigned 16-bit comparison between memory and an
immediate where the high bit is set, a clc is emitted.  This is because
the constant is created for mode HI and therefore sign extended.  This
means constraint D does not hold anymore.  Since the mode already
restricts the immediate to 16 bit, it is enough to make use of
constraint n and chop of the high bits in the output template.

gcc/ChangeLog:

	* config/s390/s390.md (*cmphi_ccu): For immediate operand 1 make
	use of constraint n instead of D and chop of high bits in the
	output template.