RFR: 8325495: C2: implement optimization for series of Add of unique value [v4]

[Kangcheng Xu]

> This pull request resolves [JDK-8325495](https://bugs.openjdk.org/browse/JDK-8325495) by converting series of additions of the same operand into multiplications. I.e., `a + a + ... + a + a + a => n*a`.
> 
> As an added benefit, it also converts `C * a + a` into `(C+1) * a` and `a << C + a` into `(2^C + 1) * a` (with respect to constant `C`). This is actually a side effect of IGVN being iterative: at converting the `i`-th addition, the previous `i-1` additions would have already been optimized to multiplication (and thus, further into bit shifts and additions/subtractions if possible).  
> 
> Some notable examples of this transformation include:
> - `a + a + a` => `a*3` => `(a<<1) + a`
> - `a + a + a + a` => `a*4` => `a<<2`
> - `a*3 + a` => `a*4` => `a<<2`
> - `(a << 1) + a + a` => `a*2 + a + a` => `a*3 + a` => `a*4 => a<<2`
> 
> See included IR unit tests for more.

Kangcheng Xu has updated the pull request incrementally with one additional commit since the last revision:

  add comments about intentional type narrowing

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20754/files
  - new: https://git.openjdk.org/jdk/pull/20754/files/30f119c5..f9ca1124

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20754&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20754&range=02-03

  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20754.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20754/head:pull/20754

PR: https://git.openjdk.org/jdk/pull/20754