S/390: Fix failing RTL check in s390_canonicalize_comparison

The new sigfpe-eh.c fails with

    internal compiler error: RTL check: expected elt 0 type 'e' or 'u', have 'w' (rtx const_int)

This is most likely due to a typo: XEXP (*op1, 0) was used, when
XEXP (*op0, 1) was intended.  This did not cause any user-visible
problems, because reversed_comparison_code_parts ignores the
respective argument, and the release compiler is built without RTL
checks.

gcc/ChangeLog:

2019-09-03  Ilya Leoshkevich  <iii@linux.ibm.com>

	* config/s390/s390.c (s390_canonicalize_comparison): Use XEXP
	(*op0, 1) instead of XEXP (*op1, 0).

gcc/testsuite/ChangeLog:

2019-09-03  Ilya Leoshkevich  <iii@linux.ibm.com>

	* gcc.target/s390/sigfpe-eh.c: New test.

From-SVN: r275336