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

	PR target/78904
	* config/i386/constraints.md (Bc): New special memory constraint.
	* config/i386/i386.md (*cmpqi_ext_1, *extvqi, *extzvqi): Use Bc
	constraint with nonimmediate_operand to allow constant memory operands.
	(*cmpqi_ext_3, insv<mode>_1, addqi_ext_1, *testqi_ext_1, andqi_ext_1)
	(*<any_or:code>qi_ext_1, *xorqi_ext_1_cc): Use Bc constraint
	with general_operand to allow constant memory operands.

testsuite/ChangeLog:

	PR target/78904
	* gcc.target/i386/pr78904-3.c: New test.

From-SVN: r243937