Fix PR rtl-optimization/102306

Fix PR rtl-optimization/102306

This is a duplication of volatile loads introduced during GCC 9 development
by the 2->2 mechanism of the RTL combiner.  There is already a substantial
checking for volatile references in can_combine_p but it implicitly assumes
that the combination reduces the number of instructions, which is of course
not the case here.  So the fix teaches try_combine to abort the combination
when it is about to make a copy of volatile references to preserve them.

gcc/
	PR rtl-optimization/102306
	* combine.c (try_combine): Abort the combination if we are about to
	duplicate volatile references.

gcc/testsuite/
	* gcc.target/sparc/20210917-1.c: New test.