RFR: 8277846: Implement fast-path for ASCII-compatible CharsetEncoders on ppc64 [v2]

[Martin Doerr]

> PPC64 port of 8274242: Implement fast-path for ASCII-compatible CharsetEncoders on x86
> I moved the `encode_iso_array` implementation into `C2_MacroAssembler` and reused it for the new ASCII node. The algorithm is unchanged. We only need to change the mask because (non-extended) ASCII uses 7 bit (also see x86 implementation).

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Enable new ASCII node.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6565/files
  - new: https://git.openjdk.java.net/jdk/pull/6565/files/96b7e6c8..b41e9d97

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6565&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6565&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6565.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6565/head:pull/6565

PR: https://git.openjdk.java.net/jdk/pull/6565