tree-optimization/107389 - honor __builtin_assume_alignment at -O0

The following makes sure to set alignment information on the LHS
of __builtin_assume_alignment calls even when not optimizing so
uses as arguments to builtin functions like memcpy or __atomic_load_n
can be reflected at RTL expansion time.

	PR tree-optimization/107389
	* gimple-low.cc (lower_builtin_assume_aligned): New.
	(lower_stmt): Call it.

	* gcc.dg/pr107389.c: New testcase.