RFR: 8249893: AARCH64: optimize the construction of the value from the bits of the other two [v6]

[Boris Ulasevich]

> Let me revive the change request [3] to C2 and AArch64 that applies Bitfield Insert instruction in the expression "(v1 & 0xFF) | ((v2 & 0xFF) << 8)". 
>  
> Compared to the last round of review [2] I updated the transformation to apply BFI in more cases and added a jtreg test.
> 
> As before, compared to the original patch [1], the transformation logic is now in the common C2 code: a new BitfieldInsert node has been introduced to replace Or+Shift+And sequence when possible, on AARCH a single BFI instruction is emitted for the new node.
>  
> [1] https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2020-July/039161.html 
> [2] https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2020-August/039653.html
> [3] https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2020-August/039792.html

Boris Ulasevich has updated the pull request incrementally with one additional commit since the last revision:

  comprehensive comment and code cleanup for BitfieldInsert transformation

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/511/files
  - new: https://git.openjdk.java.net/jdk/pull/511/files/775c3533..f210850f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=511&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=511&range=04-05

  Stats: 58 lines in 1 file changed: 31 ins; 8 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/511.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/511/head:pull/511

PR: https://git.openjdk.java.net/jdk/pull/511