[llvm] [GlobalISel] Add G_SEXT_INREG(G_ZEXT) -> G_SEXT combine. (PR #213606)

[David Green via llvm-commits]

davemgreen wrote:

Thanks - there is another transform that I was really after that performs the same transform through an unmerge, but it is more difficult to test naturally.

https://github.com/llvm/llvm-project/pull/213606