rtl-ssa: Avoid using a stale splay tree root [PR116009]

rtl-ssa: Avoid using a stale splay tree root [PR116009]

In the fix for PR115928, I'd failed to notice that "root" was used
later in the function, so needed to be updated.

gcc/
	PR rtl-optimization/116009
	* rtl-ssa/accesses.cc (function_info::add_def): Set the root
	local variable after removing the old clobber group.

gcc/testsuite/
	PR rtl-optimization/116009
	* gcc.c-torture/compile/pr116009.c: New test.