rtlanal: fix subreg handling in set_noop_p ()

rtlanal: fix subreg handling in set_noop_p ()

The following s390 rtx is errneously considered a no-op:

(set (subreg:DF (reg:TF %f0) 8) (subreg:DF (reg:V1TF %f0) 8))

Here, SET_DEST is a second register in a floating-point register pair,
and SET_SRC is the second half of a vector register, so they refer to
different bits.

Fix by treating subregs of registers in different modes conservatively.

gcc/ChangeLog:

2020-09-11  Ilya Leoshkevich  <iii@linux.ibm.com>

	* rtlanal.c (set_noop_p): Treat subregs of registers in
	different modes conservatively.