RFR: 8365147: AArch64: Replace DMB + LD + DMB with LDAR for C1 volatile field loads [v3]

[Samuel Chee]

> Replaces the DMB ISH + LD + DMB ISHLD sequence with LDAR for volatile field loads - for example, AtomicLong::get.
> 
> This is valid, as originally the DMBs were necessary due to the case described here - https://bugs.openjdk.org/browse/JDK-8179954. As in the rare case where the LD can be reordered with an LDAR or STLR from the C2 implementation for stores and loads, these DMBs are required.
> However, acquire/release operations use a sequentially consistent model which does not allow reordering between them. Hence, the LD can be replaced with an LDAR to disallow reordering with a STLR/LDAR and the first DMB can be removed.
> 
> The LDAR has acquire semantics, so it's impossible for memory accesses after to be reordered before; the DMB ISHLD is not required. Therefore, a singular LDAR is sufficient.

Samuel Chee has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Address review comments. Refine.
   
   Change-Id: I9cc0308300548c1892d39791e00b41ef13c95e63
 - Merge from the main branch
 - Address review comments
   
   Change-Id: Ica13be8094ac0f057066042ef0a5ec5927b98dfd
 - Refine code generation for mem2reg_volatile
   
   The patch is contributed by @theRealAph.
   
   Change-Id: I7ab1854dd238cdce72a4ab218b5b4ee84ad39586
 - 8365147: AArch64: Replace DMB + LD + DMB with LDAR for C1 volatile loads
   
   Replaces the DMB ISH + LD + DMB ISHLD sequence with LDAR
   for volatile field loads - for example, AtomicLong::get.
   
   This is valid, as originally the DMBs were necessary due to
   the case described here - https://bugs.openjdk.org/browse/JDK-8179954.
   As in the rare case where the LD can be reordered with an LDAR
   or STLR from the C2 implementation for stores and loads, these
   DMBs are required.
   However, acquire/release operations use a sequentially consistent model
   which does not allow reordering between them. Hence, the LD can be
   replaced with an LDAR to disallow reordering with a STLR/LDAR
   and the first DMB can be removed.
   
   The LDAR has acquire semantics, so it's impossible for
   memory accesses after to be reordered before; the DMB ISHLD is
   not required. Therefore, a singular LDAR is sufficient.
   
   This excludes floats and doubles, as they do not have
   equivalent load-acquire instructions.
   
   Change-Id: Ia93607f8bb20c2d974fe6b2e586dd3239bb2728c

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Changes: https://git.openjdk.org/jdk/pull/26748/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26748&range=02
  Stats: 111 lines in 11 files changed: 81 ins; 11 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/26748.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26748/head:pull/26748

PR: https://git.openjdk.org/jdk/pull/26748