Refactor `Expr(:simdloop)` to `Expr(:loopinfo, ...)`
- `Expr(:loopinfo, Symbol("julia.simdloop"))` replaces
`Expr(:simdloop, false)`
- `Expr(:loopinfo, Symbol("julia.simdloop"), Symbol("julia.ivdep"))` replaces
`Expr(:simdloop, true)`
While currently not directly exposed users can now pass other
[loopinfo](https://llvm.org/docs/LangRef.html#llvm-loop) metadata to
LLVM.
