combine: Take known zero bits into account when checking extraction.

gcc/ChangeLog:

	* combine.c (make_compound_operation): Take known zero bits into
	account when checking for possible zero_extend.

gcc/testsuite/ChangeLog:

	* gcc.dg/zero_bits_compound-1.c: New test.
	* gcc.dg/zero_bits_compound-2.c: New test.

From-SVN: r236813