tree-optimization/87984 - hard register assignments not preserved

The following disables redundant store elimination to hard register
variables which isn't valid.

	PR tree-optimization/87984
	* tree-ssa-dom.cc (dom_opt_dom_walker::optimize_stmt): Do
	not perform redundant store elimination to hard register
	variables.
	* tree-ssa-sccvn.cc (eliminate_dom_walker::eliminate_stmt):
	Likewise.

	* gcc.target/i386/pr87984.c: New testcase.