RFR: 8356184: C2 MemorySegment: long RangeCheck with ConvI2L(iv + invar) prevents RCE

[Kerem Kat]

`MemorySegment` bounds checks use long arithmetic, but when accessing with an int loop variable plus an int invariant offset, the pattern `ConvI2L(iv + invar)` was not recognized by Range Check Elimination. This prevented RCE and consequently blocked vectorization for common `MemorySegment` access patterns.

The fix teaches `is_scaled_iv_plus_offset` to recognize linear int expressions inside `ConvI2L`. A new `short_offset` flag signals that the offset is part of int arithmetic (not added separately in long), requiring the range to be clamped at `max_jint + 1` to correctly handle potential int overflow. This also removes pre-existing dead code where an `exp_bt != bt` check was intended to bail out on such patterns but never actually executed.

With this change, `MemorySegment` loops using int invariant offsets now benefit from RCE and vectorization, matching the behavior already supported for long invariant offsets.


void process(MemorySegment segment, int offset, int size) {
  for (int i = 0; i < size; i++) {
      long addr = i + offset;  // ConvI2L(AddI(iv, offset)) was not recognized
      segment.set(JAVA_BYTE, addr, (byte) 0);
  }
}

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Commit messages:
 - 8356184: C2 MemorySegment: long RangeCheck with ConvI2L(iv + invar) prevents RCE

Changes: https://git.openjdk.org/jdk/pull/29392/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29392&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356184
  Stats: 112 lines in 4 files changed: 82 ins; 5 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/29392.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29392/head:pull/29392

PR: https://git.openjdk.org/jdk/pull/29392