RFR: 8356246: C2: Compilation fails with "assert(bol->is_Bool()) failed: unexpected if shape" in StringConcat::eliminate_unneeded_control [v2]

[Daniel Skantz]

> This pull request contains a fix for JDK-8356246.
> 
> During stacked concatenations, a pair of `StringBuilder.append().toString()` links SB and SB2 could have a diamond if structure `(Region -> <IfTrue, IfFalse> -> If)` created by String.valueOf that depends on the return value of SB1, which is going away (replaced by top() in `eliminate_call` in stringopts).
> 
> JDK-8271341 added folding of the region of the diamond-if to stringopts to avoid the case where a live part of the graph becomes unreachable as this top() propagates through the graph too quickly.
> 
> JDK-8291775 was a follow-up fix and instead used a constant test as input to the diamond If, as a case was discovered where the If was processed before the Region leading to a broken graph.
> 
> The code in JDK-8271341 assumes that the input to the If is a boolean, not a constant. If two diamond if-region structures in the same StringBuilder candidate share the same test, the second iteration in `eliminate_unneeded_control` will fail with an unexpected input. The proposed solution is to skip over the second iteration as the test has already been replaced by a constant -- both structures will be simplified independently during IGVN.
> 
> Testing:
> T1-4.

Daniel Skantz has updated the pull request incrementally with one additional commit since the last revision:

  Apply suggestions from code review
  
  Co-authored-by: Roberto Castañeda Lozano <robcasloz at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25461/files
  - new: https://git.openjdk.org/jdk/pull/25461/files/77a8a5f4..5ae8e411

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25461&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25461&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25461.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25461/head:pull/25461

PR: https://git.openjdk.org/jdk/pull/25461