RFR: 8286451: C2: assert(nb == 1) failed: only when the head is not shared

[Roland Westrelin]

nb counts the number of loops that share a single head. The assert
that fires is in code that handles the case of a self loop (a loop
composed of a single block). There can be a self loop and multiple
loops that share a head: the assert makes little sense and I propose
to simply remove it.

I think there's another issue with this code: in the case of a self
loop and multiple loops that share a head, the self loop can be any of
the loop for which the head is cloned not only the one that's passed
as argument to ciTypeFlow::clone_loop_head(). As a consequence, I
moved the logic for self loops in the loop that's applied to all loops
that share the loop head.

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Commit messages:
 - fix & test

Changes: https://git.openjdk.java.net/jdk/pull/8947/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8947&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286451
  Stats: 79 lines in 2 files changed: 65 ins; 14 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8947.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8947/head:pull/8947

PR: https://git.openjdk.java.net/jdk/pull/8947