Add support for vector ops in the LLVM dialect

This CL is step 1/n towards building a simple, programmable and portable vector abstraction in MLIR that can go all the way down to generating assembly vector code via LLVM's opt and llc tools.

This CL adds the 3 instructions `llvm.extractelement`, `llvm.insertelement` and `llvm.shufflevector` as documented in the LLVM LangRef "Vector Instructions" section.

The "Experimental Vector Reduction Intrinsics" are left out for now and can be added in the future on a per-need basis.

Appropriate roundtrip and LLVM Target tests are added.

PiperOrigin-RevId: 262542095