RFR: 8346664: C2: Optimize mask check with constant offset [v7]

[Matthias Ernst]

> Fixes [JDK-8346664](https://bugs.openjdk.org/browse/JDK-8346664): extends the optimization of masked sums introduced in #6697 to cover constant values, which currently break the optimization.
> 
> Such constant values arise in an expression of the following form, for example from `MemorySegmentImpl#isAlignedForElement`:
> 
> 
> (base + (index + 1) << 8) & 255
> => MulNode
> (base + (index << 8 + 256)) & 255
> => AddNode
> ((base + index << 8) + 256) & 255
> 
> 
> Currently, `256` is not being recognized as a shifted value. This PR enables further reduction:
> 
> 
> ((base + index << 8) + 256) & 255
> => MulNode (this PR)
> (base + index << 8) & 255
> => MulNode (PR #6697)
> base & 255 (loop invariant)
> 
> 
> Implementation notes:
> * I verified that the originating issue "scaled varhandle indexed with i+1"  (https://mail.openjdk.org/pipermail/panama-dev/2024-December/020835.html) is resolved with this PR.
> * ~in order to stay with the flow of the current implementation, I refrained from solving general (const & mask)==0 cases, but only those where const == _ << shift.~
> * ~I modified existing test cases adding/subtracting from the index var (which would fail with current C2). Let me know if would like to see separate cases for these.~

Matthias Ernst has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 20 additional commits since the last revision:

 - Merge branch 'openjdk:master' into mernst/JDK-8346664
 - make the check more clear: shift >= mask_width
 - fully randomized
 - JLS: only the lower bits of the shift are taken into account (aka we don't assert).
 - (c)
 - (c)
 - Assert that MulNode::Ideal already masks constant shift amounts for us.
   Avoid accidental zero mask breaking test.
 - "element".
 - avoid redundant comment
 - addConstNonConstMaskLong
 - ... and 10 more: https://git.openjdk.org/jdk/compare/6144aefb...490cc2fb

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22856/files
  - new: https://git.openjdk.org/jdk/pull/22856/files/272267c2..490cc2fb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22856&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22856&range=05-06

  Stats: 70930 lines in 4052 files changed: 31120 ins; 24666 del; 15144 mod
  Patch: https://git.openjdk.org/jdk/pull/22856.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22856/head:pull/22856

PR: https://git.openjdk.org/jdk/pull/22856