[llvm] [GlobalISel] Combine (sext (trunc x)) to (sext_inreg x) (PR #131622)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

I merged it with the AArch64 test. I also restricted this to truncs >= 8 bits because that seems more sensible. Doing it on i1 truncs also causes a regression in `select_const.ll` and I don't see the usefulness of doing this for those truncs anyway.

https://github.com/llvm/llvm-project/pull/131622