RFR: JDK-8287349: Merge LDR instructions to improve C1 OSR performance

[Zhuojun Miao]

Since MacroAssembler added merge_ldst, we can use different
destination registers for contiguous-memory LDR instructions to improve performance.

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Commit messages:
 - JDK-8287349: Merge LDR instructions to improve C1 OSR performance

Changes: https://git.openjdk.java.net/jdk/pull/8933/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8933&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287349
  Stats: 3 lines in 1 file changed: 1 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8933.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8933/head:pull/8933

PR: https://git.openjdk.java.net/jdk/pull/8933