Arm64Emitter: Convert ShiftType to enum class

[Dentomologist]

Straightforward conversion of ShiftType from enum to enum class.

[BhaaLseN]

You can (probably) ignore the Windows fail, thats on our CI...but the Android one needs to be looked at. m_shifttype is now a typed enum class and doesn't implicitly convert into int, but you can just cast that inside GetData.

[Dentomologist]

I'm not sure why Visual Studio let that compile in the first place, but I've added the cast.

[lioncash]

I think it'd still be informative as LSL, LSR, ASR, and ROR as the names for the shift types. These are what they're referred to as by ARM in official documentation, and the long-form makes code (imo) slightly more verbose to read and write.

[Dentomologist]

Given that you've made roughly half the commits touching Arm64Emitter in the last few years, I'm especially inclined to defer to your judgement. I've reverted the naming changes.

Said changes were inspired by my not knowing what the terms referred to until I did some googling. In order to help anyone else in a similar situation I've added some comments in the declaration with the full name, which at least in Visual Studio appear in the hover tooltip for the respective enum values.