[m-rev.] diff: speed up ml_elim_nested

[Zoltan Somogyi]

My benchmarking shows a speedup of 3.7% on tools/speedtest,
which is funny since profiling shows that the code in question
accounts for less than 1% of the compiler's runtime. While I
don't believe the result to be accurate, it does show that the
change is unlikely to be slowdown.

Zoltan.
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