PR tree-optimization/51877
	* tree-ssa-tail-merge.c (gimple_equal_p): Don't return true whenever
	call arguments and fndecls compare equal, instead return false if they
	don't.  Return true only if lhs1 and lhs2 are either both NULL, or
	both SSA_NAMEs that are valueized the same, or they satisfy
	operand_equal_p.

	* gcc.c-torture/execute/pr51877.c: New test.

From-SVN: r183237