This changes `MCRegUnit` type from `unsigned` to `enum class : unsigned`
and inserts necessary casts.
The added `MCRegUnitToIndex` functor is used with `SparseSet`,
`SparseMultiSet` and `IndexedMap` in a few places.

`MCRegUnit` is opaque to users, so it didn't seem worth making it a
full-fledged class like `Register`.

Static type checking has detected one issue in
`PrologueEpilogueInserter.cpp`, where `BitVector` created for
`MCRegister` is indexed by both `MCRegister` and `MCRegUnit`.

The number of casts could be reduced by using `IndexedMap` in more
places and/or adding a `BitVector` adaptor, but the number of casts *per
file* is still small and `IndexedMap` has limitations, so it didn't seem
worth the effort.

Pull Request: https://github.com/llvm/llvm-project/pull/167943

Not all RegisterId values are registers, so Id is a more appropriate
name.

Use asMCReg() in some places that assumed it was a register.

RegisterId can represent a physical register, a MCRegUnit, or
an index into a side structure that stores register masks. These 3
types were encoded by using the physical reg, stack slot, and
virtual register encoding partitions from the Register class.
This encoding scheme alias wasn't well contained so
Register::index2StackSlot and Register::stackSlotIndex appeared
in multiple places.

This patch gives RegisterRef its own encoding defines and separates
it from Register.

I've removed the generic idx() method in favor of getAsMCReg(),
getAsMCRegUnit(), and getMaskIdx() for some degree of type safety.

Some places used the RegisterId field of RegisterRef directly as a
register. Those have been updated to use getAsMCReg.
    
Some special cases for RegisterId 0 have been removed as it can
be treated like a MCRegister by existing code.
    
I think I want to rename the Reg field of RegisterRef to Id, but
I'll do that in another patch.
    
Additionally, callers of the RegisterRef constructor need to be
audited for implicit conversions from Register/MCRegister
to unsigned.

Migrate the few users to the nonstatic member functions.

This simplifies every use of MCRegUnitMaskIterator.

Differential Revision: https://reviews.llvm.org/D157864

Differential Revision: https://reviews.llvm.org/D153122

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D152098