combine: Don't replace SET_SRC with REG_EQUAL note content if SET_SRC has side-effect...

combine: Don't replace SET_SRC with REG_EQUAL note content if SET_SRC has side-effects [PR94873]

There were some discussions about whether REG_EQUAL notes are valid on insns with a single
set which contains auto-inc-dec side-effects in the SET_SRC and the majority thinks that
it should be valid.  So, this patch fixes the combiner to punt in that case, because otherwise
the auto-inc-dec side-effects from the SET_SRC are lost.

2020-05-06  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/94873
	* combine.c (combine_instructions): Don't optimize using REG_EQUAL
	note if SET_SRC (set) has side-effects.

	* gcc.dg/pr94873.c: New test.