Follow-up on slice arguments

[fruffy]

I started fuzzing slices again after the recent fix to #2147 (hurray). Unfortunately, there seem to be more nasty edge cases.

In this action we assign 2 (0000 0010) to h.h.a while also assigning 1 to the least significant bit. The final value of h.h.a should be 3 (0000 0011). However, in 35_SimplifyDefUse, the compiler removes the assignment to h.h.a. Copy-out should only copy back the least significant bit, but it looks like the pass assumes that the entire header is copied back. At least that is how I understand it.

control ingress(inout Headers h, inout Meta m, inout standard_metadata_t sm) {
    @name("slice_action") action slice_action_0(inout bit<1> sliced_val) {
        h.h.a = 8w2;
        sliced_val = 1w1;
    }
    apply {
        slice_action_0(h.h.a[0:0]);
    }
}

to

control ingress(inout Headers h, inout Meta m, inout standard_metadata_t sm) {
    @name("slice_action") action slice_action_0(inout bit<1> sliced_val) {
        ;
        sliced_val = 1w1;
    }
    apply {
        slice_action_0(h.h.a[0:0]);
    }
}

slice_assignment.p4.txt
slice_assignment.stf.txt