bpf: fix memory constraint of ldx/stx instructions [PR108790]

bpf: fix memory constraint of ldx/stx instructions [PR108790]

In some cases where the target memory address for an ldx or stx
instruction could be reduced to a constant, GCC could emit a malformed
instruction like:

    ldxdw %r0,0

Rather than the expected form:

    ldxdw %rX, [%rY + OFFSET]

This is due to the constraint allowing a const_int operand, which the
output templates do not handle.

Fix it by introducing a new memory constraint for the appropriate
operands of these instructions, which is identical to 'm' except that
it does not accept const_int.

gcc/

	PR target/108790
	* config/bpf/constraints.md (q): New memory constraint.
	* config/bpf/bpf.md (zero_extendhidi2): Use it here.
	(zero_extendqidi2): Likewise.
	(zero_extendsidi2): Likewise.
	(*mov<MM:mode>): Likewise.

gcc/testsuite/

	PR target/108790
	* gcc.target/bpf/ldxdw.c: New test.