[MCP][NFC] Opinionated refactoring (#186239)

There are a few minor inconsistencies across the pass which I found mildly distracting:

* The use of `Def`/`Dest`/`Dst` to refer to the same thing
* Inconsistent declaration order of `Dst`/`Src` vs `Src`/`Dst`
* Lots of `->getReg()->asMCReg()`, and uses of `Register` when the pass
  is always running after RA anyway.
* Some places explicitly `assert(isCopyInstr)` while others just deref
  the `optional`.

Standardize on `Dst`/`Src` to match the metaphor and ordering of
`DestSourcePair`.

Assume `std::optional::operator*` will assert in any reasonable
implementation, even though this may technically be undefined behavior.
When asserts are disabled it would be anyway.

The refactor uses structured bindings for a couple reasons:

* Naturally enforces consistent order of `Dst`-then-`Src`
* Requires the use of `auto`, which ensures the declaration is not
  implicitly converting from `MCRegister` back to `Register`.

In both cases the explicitness of the name `getDstSrcMCRegs` hopefully
makes the meaning at the callsite clear (`Dst, Src = DstSrc`, and
explicitly mentioning `MCReg`).

Change-Id: Ic58f555e03535d726cdad38dbe3f9c6df1b86460

Author: slinder1