RFR: 8353041: NeverBranchNode causes incorrect block frequency calculation

[Roberto Castañeda Lozano]

On Wed, 2 Apr 2025 19:51:37 GMT, Dean Long <dlong at openjdk.org> wrote:

> This fixes a quality of implementation issue for infinite loops using a NeverBranch node.  We need Block::succ_prob() to return 1.0f for the 100% successful back-edge so that block frequencies are computed correctly.  I also fixed Block_Stack::most_frequent_successor() to choose the correct successor.  I verified that this corrects the huge frequency ratio that was detected and clamped by JDK-8346888.
> 
> Currently this bug is labeled noreg-hard with no new regression test, as it's not obvious how to write such as test.

Marked as reviewed by rcastanedalo (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/24390#pullrequestreview-2749204166