s390: Fix s390_canonicalize_comparison

Combine may come up with insns of the form:

Trying 230 -> 231:
  230: %cc:CCZ=unspec[r82:DF,0xfc0]TDC_INSN
  231: r83:SI=unspec[%cc:CCZ]CC_TO_INT
      REG_DEAD %cc:CCZ
Failed to match this instruction:
(set (reg:SI 83 [ _38 ])
    (unspec:SI [
            (unspec:CCZ [
                    (reg:DF 82 [ _37 ])
                    (const_int 4032 [0xfc0])
                ] UNSPEC_TDC_INSN)
        ] UNSPEC_CC_TO_INT))

where the operand for UNSPEC_CC_TO_INT is not a register.  Deal with
this during s390_canonicalize_comparison.

While on it canonicalize GET_CODE (x) == REG to REG_P (x).

gcc/ChangeLog:

	* config/s390/s390.cc (s390_canonicalize_comparison): Verify op0
	for an UNSPEC_CC_TO_INT is a register.