* rtlanal.c (subreg_lsb_1): New function split out from subreg_lsb.
	(subreg_lsb): Change to call new subreg_lsb_1 helper function.
	* rtl.h (subreg_lsb_1): Prototype here.
	* simplify-rtx.c (simplify_subreg): Optimize subregs of zero and
	sign extensions.

Co-Authored-By: Paolo Bonzini <bonzini@gnu.org>

From-SVN: r76352