[m-rev.] for post-commit review: improve debug code in polymorphism_type_class_info.m

[Zoltan Somogyi]

This helped me to track down the source of one of the problems
in the tci_liveness_abort test case: the creation of a stray base typeclass
info. As the attached trace output file (for a cutdown version of that
test case) shows,

- we generate the same value as the base typeclass info for both
  foo(list(T)) and foo(list(list(T))), but

- we do not reuse the variable holding the first base typeclass info
  when we need the second, because we look for them in different
  "slots" in the typeclass info map.

This diff makes no algorithmic changes in non-debug code; that is
for later diffs.

For review by anyone.

Zoltan.
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