PR rtl-optimization/56494
	* simplify-rtx.c (simplify_truncation): If C is narrower than A,
	optimize (truncate:A (subreg:B (truncate:C X) 0)) into
	(subreg:A (truncate:C X) 0) instead of (truncate:A X).

	* gcc.dg/pr56494.c: New test.

From-SVN: r196451