RFR: 8256823: C2 compilation fails with "assert(isShiftCount(imm8 >> 1)) failed: illegal shift count" [v4]

[Tobias Hartmann]

> The ideal transformation added by [JDK-8254872](https://bugs.openjdk.java.net/browse/JDK-8254872) converts `RotateLeftNode(val, shift)` into `RotateRightNode(val, 32/64 - (shift & 31/63))`. If `shift` later becomes zero, we end up trying to emit a rotate with a 32/64 shift value which triggers an assert.
> 
> I've added an identity transformation similar to what is implemented for ShiftNodes that takes care of this. I've also noticed that the corresponding assert is not strong enough for `roll` and `rorl` (probably the author used the assert corresponding to the 64-bit version by accident). The patch also includes some refactoring.
> 
> Thanks,
> Tobias

Tobias Hartmann has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merged with master
 - Style cleanup in mulnode.cpp
 - Refactored getShiftCon method
 - 8256823: C2 compilation fails with "assert(isShiftCount(imm8 >> 1)) failed: illegal shift count"

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Changes: https://git.openjdk.java.net/jdk/pull/1384/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1384&range=03
  Stats: 185 lines in 5 files changed: 120 ins; 5 del; 60 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1384.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1384/head:pull/1384

PR: https://git.openjdk.java.net/jdk/pull/1384