[opt] Add -mtune option (#187932)
This patch adds a Clang-compatible -mtune option to opt, following up
the addition to llc: https://github.com/llvm/llvm-project/pull/186998,
to enable decoupled ISA and microarchitecture targeting. Example use
case is `MaxInterleaveFactor` for the loop vectorizer.
The implementation registers the new codegen flag for opt, which
consumes it and sets tune-cpu attributes for functions to be consumed by
the backend.
