VAX: Check the correct operand for constant 0 push operation

VAX: Check the correct operand for constant 0 push operation

Check the output operand for representing pushing a value onto the stack
rather than the constant 0 input in determining whether to use the PUSHL
or the CLRL instruction for a SImode move.  The latter actually works by
means of using the predecrement addressing mode with the SP register and
the machine code produced even takes the same number of bytes, however
at least with some VAX implementations it incurs a performance penalty.
Besides, we don't want to check the wrong operand anyway and have code
that works by chance only.

Add a test case covering push operations; for operands different from
constant zero there is actually a code size advantage for using PUSHL
rather than the equivalent MOVL instruction.

	gcc/
	* config/vax/vax.c (vax_output_int_move): Check the correct
	operand for constant 0 push operation.

	gcc/testsuite/
	* gcc.target/vax/push.c: New test.