RFR: 8346664: C2: Optimize mask check with constant offset [v6]

[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 incrementally with one additional commit since the last revision:

  make the check more clear: shift >= mask_width

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22856/files
  - new: https://git.openjdk.org/jdk/pull/22856/files/4849477f..272267c2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22856&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22856&range=04-05

  Stats: 13 lines in 1 file changed: 6 ins; 0 del; 7 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