[llvm] [GlobalIsel] Cleanup G_EXTRACT_VECTOR_ELT combines (PR #109047)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Ok, you got me. It is indeed non-nfc changes. It just did not show up in the tests. Bad test coverage, blame me.

`getIConstantFromReg` is the new compromise between avoiding optional and hiding implementation details. I known that in the pattern is a G_CONSTANT on the register.

https://github.com/llvm/llvm-project/pull/109047