RFR: 8300257: C2: vectorization fails on some simple Memory Segment loops

[Roland Westrelin]

In the test case `testByteLong1` (that's extracted from a memory
segment micro benchmark), the address of the store is initially:


(AddP#204 base#195 base#195 (AddL#164 (ConvI2L#158 (CastII#157 (LshiftI#107 iv#101))) invar#163))


(#numbers are node numbers to help the discussion).

`iv#101` is the `Phi` of a counted loop. `invar#163` is the
`baseOffset` load.

To eliminate the range check, the loop is transformed into a loop nest
and as a consequence the address above becomes:


(AddP#204 base#195 base#195 (AddL#164 (ConvI2L#158 (CastII#157 (LShiftI#107 (AddI#326 invar#308 iv#321)))) invar#163))


`invar#308` is some expression from a `Phi` of the outer loop.

That `AddP` is transformed multiple times to push the invariants out of loop:


(AddP#568 base#195 (AddP#556 base#195 base#195 invar#163) (ConvI2L#158 (CastII#157 (AddI#566 (LShiftI#565 iv#321) invar#577))))


then:


(AddP#568 base#195 (AddP#847 (AddP#556 base#195 base#195 invar#163) (AddL#838 (ConvI2L#793 (LShiftL#760 iv#767)) (ConvI2L#818 (CastII#779 invar#577)))))


and finally:


(AddP#568 base#195 (AddP#949 base#195 (AddP#855 base#195 (AddP#556 base#195 base#195 invar#163) (ConvI2L#818 (CastII#809 invar#577))) (ConvI2L#938 (LShiftI#896 iv#908))))


`AddP#855` is out of the inner loop. 

This doesn't vectorize because:

- there are 2 invariants in the address expression but superword only
  support one (tracked by `_invar` in `SWPointer`)

- there are more levels of `AddP` (4) than superword supports (3)

To fix that, I propose to no longer track the address elements in
`_invar`, `_negate_invar` and `_invar_scale` but instead to have a
single `_invar` which is an expression built by superword as it
follows chains of `addP` nodes. I kept the previous `_invar`,
`_negate_invar` and `_invar_scale` as debugging and use them to check
that what vectorized with the previous scheme still does.

I also propose lifting the restriction on 3 levels of `AddP` entirely.

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

Commit messages:
 - fix & test

Changes: https://git.openjdk.org/jdk/pull/12942/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12942&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300257
  Stats: 274 lines in 3 files changed: 212 ins; 23 del; 39 mod
  Patch: https://git.openjdk.org/jdk/pull/12942.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12942/head:pull/12942

PR: https://git.openjdk.org/jdk/pull/12942