Validate c1 arraycopy destination registry

While working on JDK-8302850 I ended up writing assembly code with this shape:

```
void LIRGenerator::do_Clone(Intrinsic* x) {
    assert(x->number_of_arguments() == 1, "wrong type");

    CodeEmitInfo* info = state_for(x, x->state());

    LIRItem src(x->argument_at(0), this);
    ciType* src_type = src.value()->exact_type();

    LIRItem array(src.value(), this);
    LIR_Opr len = FrameMap::r19_opr;
    __ load(new LIR_Address(array.result(), arrayOopDesc::length_offset_in_bytes(), T_INT), len, info, lir_patch_none);

    LIR_Opr reg = result_register_for(x->type());
    LIR_Opr tmp1 = FrameMap::r10_oop_opr;
    LIR_Opr tmp2 = FrameMap::r11_oop_opr;
    LIR_Opr tmp3 = FrameMap::r5_oop_opr;
    LIR_Opr tmp4 = reg;
    LIR_Opr klass_reg = FrameMap::r3_metadata_opr;
    ciArrayKlass* array_klass = src_type->as_array_klass();
    BasicType elem_type = array_klass->element_type()->basic_type();

    __ metadata2reg(ciTypeArrayKlass::make(elem_type)->constant_encoding(), klass_reg);

    info = state_for(x, x->state()); // TODO is this right?
    CodeStub* slow_path = new NewTypeArrayStub(klass_reg, len, reg, info);
    __ allocate_array(reg, len, tmp1, tmp2, tmp3, tmp4, elem_type, klass_reg, slow_path);

    LIRItem src_pos(new Constant(new IntConstant(0)), this);
    LIR_Opr dst = FrameMap::as_opr(j_rarg2);
    __ move(reg, dst);
    LIRItem dst_pos(new Constant(new IntConstant(0)), this);
    LIR_Opr length = FrameMap::as_opr(j_rarg4);

    // operands for arraycopy must use fixed registers, otherwise
    // LinearScan will fail allocation (because arraycopy always needs a
    // call)

    // The java calling convention will give us enough registers
    // so that on the stub side the args will be perfect already.
    // On the other slow/special case side we call C and the arg
    // positions are not similar enough to pick one as the best.
    // Also because the java calling convention is a "shifted" version
    // of the C convention we can process the java args trivially into C
    // args without worry of overwriting during the xfer

    src.load_item_force (FrameMap::as_oop_opr(j_rarg0));
    src_pos.load_item_force (FrameMap::as_opr(j_rarg1));
    // __ move(dst, FrameMap::as_opr(j_rarg2));
    dst_pos.load_item_force (FrameMap::as_opr(j_rarg3));
    __ move(len, length);
    LIR_Opr tmp = FrameMap::as_opr(j_rarg5);

    int flags = 0;
    ciArrayKlass* expected_type = src_type->as_array_klass();

    __ arraycopy(src.result(), src_pos.result(), FrameMap::as_opr(j_rarg2), dst_pos.result(), length, tmp, expected_type, flags, info); // does add_safepoint

    LIR_Opr result = rlock_result(x);
    __ move(dst, result);
}
```

Running that results in a sigsegv on x0 due to the result being in the destination registry `j_arg2`. I discussed this with Roland who pointed out that:

{quote}
I think the bug is that when you passed j_rarg2 to array_copy and use it afterwards, nothing tells you're doing something impossible. c1 knows j_rarg2 is killed by array_copy.
{quote}

The register allocator code should assert this rather than let it through and waiting for the runtime sigsegv.

As a FYI, the issue is avoided if dst is placed in a different register. Here's an example that Roland suggested:

```
    LIR_Opr dst = new_register(T_OBJECT);
    __ move(reg, dst);
    ...
    __ move(dst, FrameMap::as_opr(j_rarg2));
    ...
    __ arraycopy(...);
    ...
    __ move(dst, result);
```