RFR: 8347459: C2: missing transformation for chain of shifts/multiplications by constants [v16]

[Marc Chevalier]

> This collapses double shift lefts by constants in a single constant: (x << con1) << con2 => x << (con1 + con2). Care must be taken in the case con1 + con2 is bigger than the number of bits in the integer type. In this case, we must simplify to 0.
> 
> Moreover, the simplification logic of the sign extension trick had to be improved. For instance, we use `(x << 16) >> 16` to convert a 32 bits into a 16 bits integer, with sign extension. When storing this into a 16-bit field, this can be simplified into simple `x`. But in the case where `x` is itself a left-shift expression, say `y << 3`, this PR makes the IR looks like `(y << 19) >> 16` instead of the old `((y << 3) << 16) >> 16`. The former logic didn't handle the case where the left and the right shift have different magnitude. In this PR, I generalize this simplification to cases where the left shift has a larger magnitude than the right shift. This improvement was needed not to miss vectorization opportunities: without the simplification, we have a left shift and a right shift instead of a single left shift, which confuses the type inference.
> 
> This also works for multiplications by powers of 2 since they are already translated into shifts.
> 
> Thanks,
> Marc

Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:

  Rephrase comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23728/files
  - new: https://git.openjdk.org/jdk/pull/23728/files/e3ecf350..124d9382

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23728&range=15
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23728&range=14-15

  Stats: 52 lines in 1 file changed: 18 ins; 6 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/23728.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23728/head:pull/23728

PR: https://git.openjdk.org/jdk/pull/23728