RFR: 8353192: C2: Clean up x86 backend after 32-bit x86 removal

[Aleksey Shipilev]

Piece-wise cleanup of C2 x86 backend. C2_MacroAssembler, x86.ad and related files are the target for this cleanup. 

Additional testing:
 - [ ] Linux x86_64 server fastdebug, `all`
 - [ ] Linux x86_64 server fastdebug, `all` + `-XX:-TieredCompilation`

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Commit messages:
 - Touchup
 - Fix

Changes: https://git.openjdk.org/jdk/pull/24300/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24300&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353192
  Stats: 511 lines in 5 files changed: 1 ins; 439 del; 71 mod
  Patch: https://git.openjdk.org/jdk/pull/24300.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24300/head:pull/24300

PR: https://git.openjdk.org/jdk/pull/24300