[llvm] [llvm][ARM]Add ARM widen strings pass (PR #107120)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

If we're going to make this transform target-independent, we'll need some target-specific tuning from TargetTransformInfo or something like that.  Even if the transform is profitable, the exact thresholds where it's profitable are likely to be different.  (The maximum size of the global where it's relevant, and whether the best alignment boundary is 2/4/8/16 bytes, is going to vary.)

Not sure we need extensive performance measurements for other targets... if you could get measurements for some big x86 or Arm64 core, that would be nice.  But you can basically see what happens on other targets by just compiling a simple example.  And if we have a TTI hook, targets could easily opt-out.

https://github.com/llvm/llvm-project/pull/107120