[mlir][LLVMIR] Add support for translating from some simple LLVM instructions

Add support for translating from llvm::Select, llvm::FNeg, and llvm::Unreachable.
This patch also cleans up (NFC) the opcode map for simple instructions and
adds `// clang-format off/on` comments to prevent those lines from being
churned by clang-format between commits.

Differential Revision: https://reviews.llvm.org/D125817