Loop refactoring and commenting improvements (#61496)

Loop refactoring and commenting improvements (#61496)

- Remove unneeded FIRST concept in loop table; it was always equal to TOP.
- Rename optMarkLoopsBlocks to optScaleLoopBlocks to more accurately describe
what it does. More consistently report block scaling in the dump
- Create optMarkLoopHeads. This was refactored out of fgRemoveUnreachableBlocks so
it can be called in a more logical location (at the beginning of optFindLoops),
and only does one thing.
- fgMakeOutgoingStructArgCopy: remove unused `argIndex` argument; reorder calls to
fgMightHaveLoop.
- Update and write a bunch of comments; convert away from `/* */` style comments.