Introduce enum for BasicBlock and loop flags (#52410)

Introduce enum for BasicBlock and loop flags (#52410)

* Introduce enum for BasicBlock and loop flags

This gives a better debugging experience in Visual Studio. It
also improves type checking: there were a few places still using
`unsigned` instead of `unsigned __int64` when manipulating
BasicBlock flags.

* Make sure debugreturn and contracts are disabled for the JIT build

* Convert GenTree flags, debug flags, and call flags to enums

* Remove bad GT_HWINTRINSIC copy/paste code from GenTree::GetRegSpillFlagByIdx