Fix incorrect copy propagation into table match keys

[vhavel]

If table is applied at multiple places, local copy propagation must ensure expressions are propagated only if all control flow paths to all apply method calls assign the same expression to a variable. Currently, LocalCopyPropagation correctly inhibits copy propagation into match key k if two different paths to apply() call assign a different (not equivalent) expressions into k. But if one path does not assign any value to k, and all other assign k=a, e.g.:

if (cond) {
    k = a;
    t.apply();
} else {
    t.apply();
}

the current implementation incorrectly propagates a into the table's match key (rewriting k to a).

This PR fixes this problem by forbidding copy propagation of a match key in presence of a path without any remap. Before this change, the testcase testdata/p4_16_samples/copyprop1.p4 fails because copyprop1-midend.p4 incorrectly matches on hdr.payload.y instead of x.

[mihaibudiu]

@ChrisDodd wrote this code, so he should review this PR.