RFR: 8277850: C2: optimize mask checks in counted loops [v2]

[Roland Westrelin]

> This is another fix that addresses a performance issue with panama and that was brought up by Maurizio. The pattern to optimize is:
> if ((base + (offset << 2)) & 3) != 0) {
> }
> 
> where base is loop independent but offset depends on a loop variable. This can be transformed to:
> 
> if ((base & 3) != 0) {
> 
> That check becomes loop independent and be optimized by loop predication (or I suppose loop unswitching but that wasn't the case of the micro benchmark I worked on).
> 
> This change also optimizes the pattern:
> 
> (offset << 2) & 3
> 
> to return 0.

Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - reviews
 - Merge branch 'master' into JDK-8277850
 - whitespace
 - fix
 - fix

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6697/files
  - new: https://git.openjdk.java.net/jdk/pull/6697/files/0d0a7c9c..f7165648

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6697&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6697&range=00-01

  Stats: 11458 lines in 536 files changed: 7406 ins; 1974 del; 2078 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6697.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6697/head:pull/6697

PR: https://git.openjdk.java.net/jdk/pull/6697