[m-rev.] diff: improve simplify_info

[Zoltan Somogyi]

I speedtested versions of this diff as I made the changes listed in
the log message, and got results that tools/speed_summary reported as
ranging from a 5% speedup to no change at all. I don't believe either
of those extremes; the truth is very likely to be a speedup of between
0.5 to 1%. Unfortunately, the effectively random operation of
thermal throttling makes consistent benchmark timings very hard
to achieve. (For just one executable, the final version, its raw times
for compiling the 50 biggest compiler modules ranged from 29.x seconds
to 36.x seconds.)

Zoltan.
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