[11u] 8242429: Better implementation for sign extract

[Dmitry Chuyko]

Hello,

Original bug: https://bugs.openjdk.java.net/browse/JDK-8242429

Original patch applies almost cleanly except the copyright year update 
in mulnode.cpp - re-done.

11u webrev: http://cr.openjdk.java.net/~dchuyko/8242429/webrev.11u.00/

Testing: compiler/c2/TestSignExtract.java, tier1, tier2 (aarch64 build).

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Thanks,
-Dmitry