handle instructions that read and write different parts of the same instruction

the motivating case is `xchg ah, al`, where both register writes
independently "don't match" the overall register diff of the low 16
bits. the diff-checking code was too narrow: we really have to collect
all allowed diffs on a register for an instruction and compare the
actual diff to that unification.

the implementation goes the other way though: compute the diff, and
remove parts of the diff that are unaccounted for. if any diff remains,
that is by definition unexpected and an error.

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