[llvm] [GlobalISel] Handle div-by-pow2 (PR #83155)

[Shilei Tian via llvm-commits]

shiltian wrote:

> > After taking the advices, the code generated from GlobalISel was not changed for the case above. Need to figure out where they come from, especially the compare instruction.
> 
> You are still generating too much instructions. For example,
> 
> ```
>  auto C1 = Builder.buildCTTZ(Ty, RHS);
> ```
> 
> `RHS` is a constant, and you have the value in `RHSC`. So instead of generating a `G_CTTZ` instruction, take the constant value (`APInt Cst = RHSC->Value;`), calculate the result, and use the resulting constant in the next step. Same with the `G_SUB` for the inexact value.

This does it. Thanks! It looks like there is no sort of "constant folding" after this pass.

https://github.com/llvm/llvm-project/pull/83155