re PR target/78904 (zero-extracts are not effective)

	PR target/78904
	* config/i386/constraints.md (Bn): New special memory constraint.
	* config/i386/predicates.md (norex_memory_operand): New predicate.
	* config/i386/i386.md (*extzvqi_mem_rex64): New insn pattern and
	corresponding peephole2 pattern.

testsuite/ChangeLog:

	PR target/78904
	* gcc.target/i386/pr78904-4.c: New test.
	* gcc.target/i386/pr78904-5.c: Ditto.

From-SVN: r243955