RFR: JDK-8316991: Reduce nullable allocation merges [v3]

[Cesar Soares Lucas]

> ### Description
> 
> Many, if not most, allocation merges (Phis) are nullable because they join object allocations with "NULL", or objects returned from method calls, etc. Please review this Pull Request that improves Reduce Allocation Merge implementation so that it can reduce at least some of these allocation merges.
> 
> Overall, the improvements are related to 1) making rematerialization of merges able to represent "NULL" objects, and 2) being able to reduce merges used by CmpP/N and CastPP. 
> 
> The approach to reducing CmpP/N and CastPP is pretty similar to that used in the `MemNode::split_through_phi` method: a clone of the node being split is added on each input of the Phi. I make use of `optimize_ptr_compare` and some type information to remove redundant CmpP and CastPP nodes. I added a bunch of ASCII diagrams illustrating what some of the more important methods are doing.
> 
> ### Benchmarking
> 
> **Note:** In some of these tests no reduction happens. I left them in to validate that no perf. regression happens in that case.
> **Note 2:** Marging of error was negligible.
> 
> | Benchmark                            |  No RAM (ms/op)  |   Yes RAM (ms/op) |
> |--------------------------------------|------------------|-------------------|
> | TestTrapAfterMerge                   |      19.515      |      13.386       |
> | TestArgEscape                        |      33.165      |      33.254       |
> | TestCallTwoSide                      |      70.547      |      69.427       |
> | TestCmpAfterMerge                    |      16.400      |       2.984       |
> | TestCmpMergeWithNull_Second          |      27.204      |      27.293       |
> | TestCmpMergeWithNull                 |       8.248      |       4.920       |
> | TestCondAfterMergeWithAllocate       |      12.890      |       5.252       |
> | TestCondAfterMergeWithNull           |       6.265      |       5.078       |
> | TestCondLoadAfterMerge               |      12.713      |       5.163       |
> | TestConsecutiveSimpleMerge           |      30.863      |       4.068       |
> | TestDoubleIfElseMerge                |      16.069      |       2.444       |
> | TestEscapeInCallAfterMerge           |      23.111      |      22.924       |
> | TestGlobalEscape                     |      14.459      |      14.425       |
> | TestIfElseInLoop                     |     246.061      |      42.786       |
> | TestLoadAfterLoopAlias               |      45.808      |      45.812       |
> | TestLoadAfterTrap                    |      28.370      |   ...

Cesar Soares Lucas has updated the pull request incrementally with one additional commit since the last revision:

  Refrain from RAM of arrays and Phis controlled by Loop nodes.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15825/files
  - new: https://git.openjdk.org/jdk/pull/15825/files/e8e9c13d..257e0447

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15825&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15825&range=01-02

  Stats: 7 lines in 1 file changed: 5 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15825.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15825/head:pull/15825

PR: https://git.openjdk.org/jdk/pull/15825