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

Bug relates to `early-if-predicator` and `early-ifcvt` passes. If
virtual register has "killed" flag in both basic blocks to be merged
into head, both instructions in head basic block will have "killed" flag
for this register. It makes MIR incorrect.

Example:

```
  bb.0: ; if
    ...
    %0:intregs = COPY $r0
    J2_jumpf %2, %bb.2, implicit-def dead $pc
    J2_jump %bb.1, implicit-def dead $pc

  bb.1: ; if.then
    ...
    S4_storeiri_io killed %0, 0, 1
    J2_jump %bb.3, implicit-def dead $pc

  bb.2: ; if.else
    ...
    S4_storeiri_io killed %0, 0, 1
    J2_jump %bb.3, implicit-def dead $pc
```

After early-if-predicator will become:

```
  bb.0:
    %0:intregs = COPY $r0
    S4_storeirif_io %1, killed %0, 0, 1
    S4_storeirit_io %1, killed %0, 0, 1
```

Having `killed` flag set twice in bb.0 for `%0` is an incorrect MIR.

This reverts commit ff99af7ea03b3be46bec7203bd2b74048d29a52a.

Use nonstatic member instead. This requires explicit conversions, but
many will go away as we continue converting unsigned to Register.

In a few places where it was simple, I changed unsigned to Register.

Support `MachineFunction` in `OptNoneInstrumentation`, also add
`isRequired` to all necessary passes.