Update check for llvm.vector.splice

[PietroGhg]

Overview

llvm.vector.splice is no longer experimental in LLVM 19, we need to update our lit tests.

Reason for change

Lit tests that check for llvm.experimental.vector.splice are failing.

Description of change

Update lit tests to optionally match .experimental.

Anything else we should know?

If there's any other relevant information we should know that may help us in understanding and verifying your patch, please include it here.

Checklist

• Read and follow the project Code of Conduct.
• Make sure the project builds successfully with your changes.
• Run relevant testing locally to avoid regressions.
• Run clang-format-17 on all modified code.