RFR: 8263075: C2: simplify anti-dependence check in PhaseCFG::implicit_null_check() [v3]

[Brian J. Stafford]

> The reporter for this issue (https://bugs.openjdk.java.net/browse/JDK-8263075) indicated that there's an assumption that we can rely on that the while loop in question will run exactly one time. Based on this, I've done the following:
> 
> - Asserted the condition that makes sure the code runs at least once
> - Asserted the condition that makes sure the code runs only once
> - Removed the `while` loop
> - Changed a couple of `break` statements into `continue` statements. They no longer need to break out of the `while` loop, now that it's gone. However, they were early exits from the `while` loop that ended up resulting in `continue` statements for the larger enclosing loop. Thus we can just call `continue` directly.
> - Removed the local variable `b`, as we no longer need to traverse the node hierarchy. We can use `mb` directly.
> 
> Passes jdk, langtools, and hotspot Tier 1 tests on Linux (x64 and ARM64) and macOS (x64 and ARM64). Most Tier 1 tests pass on Windows (x64 and ARM64), but there are a handful of failures unrelated to this change.

Brian J. Stafford has updated the pull request incrementally with one additional commit since the last revision:

  Updated based on PR feedback

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8684/files
  - new: https://git.openjdk.java.net/jdk/pull/8684/files/c055174a..8129fe21

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8684&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8684&range=01-02

  Stats: 14 lines in 1 file changed: 3 ins; 6 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8684.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8684/head:pull/8684

PR: https://git.openjdk.java.net/jdk/pull/8684