RFR: 8266561: Remove Compile::_save_argument_registers

[Claes Redestad]

Compile::_save_argument_registers is always false, so I suggest removing it.

It was used in the past for certain stubs, but the last use appears to have been removed with [JDK-8136406](https://bugs.openjdk.java.net/browse/JDK-8136406)

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Commit messages:
 - Minor cleanups
 - C->save_argument_registers() is effectively always false and can be removed

Changes: https://git.openjdk.java.net/jdk/pull/3884/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3884&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266561
  Stats: 65 lines in 8 files changed: 0 ins; 33 del; 32 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3884.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3884/head:pull/3884

PR: https://git.openjdk.java.net/jdk/pull/3884