combine: Fix up simplify_shift_const_1 for nested ROTATEs [PR97386]

combine: Fix up simplify_shift_const_1 for nested ROTATEs [PR97386]

The following testcases are miscompiled (the first one since my improvements
to rotate discovery on GIMPLE, the other one for many years) because
combiner optimizes nested ROTATEs with narrowing SUBREG in between (i.e.
the outer rotate is performed in shorter precision than the inner one) to
just one ROTATE of the rotated constant.  While that (under certain
conditions) can work for shifts, it can't work for rotates where we can only
do that with rotates of the same precision.

2020-10-13  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/97386
	* combine.c (simplify_shift_const_1): Don't optimize nested ROTATEs if
	they have different modes.

	* gcc.c-torture/execute/pr97386-1.c: New test.
	* gcc.c-torture/execute/pr97386-2.c: New test.