RFR: 8347645: C2: XOR bounded value handling blocks constant folding [v8]

[Johannes Graham]

> C2 does not eliminate XOR nodes with constant arguments. This has a noticeable effect on `Long.expand` with a constant mask, on architectures that don't have instructions equivalent  to `PDEP` to be used in an intrinsic.
> 
> This patch demonstrates a potential fix to the problem, but there might well be better ways to do it.

Johannes Graham has updated the pull request incrementally with two additional commits since the last revision:

 - test cleanup, stub of a test with gtest
 - clean up comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23089/files
  - new: https://git.openjdk.org/jdk/pull/23089/files/cafdbfd7..db16d38c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23089&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23089&range=06-07

  Stats: 60 lines in 4 files changed: 37 ins; 1 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/23089.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23089/head:pull/23089

PR: https://git.openjdk.org/jdk/pull/23089