[DebugInfo][InstrRef][NFC] Refactor ahead of further optimisations
This patch shuffles some functions around so that some blocks of code can
be reused. In particular,
* Move the determination of "which blocks are in scope" to its own
function, as it's non-trivial to solve. Delete the "InScopeBlocks"
collection too, which nothing reads from.
* Split transfer emission (i.e., installing DBG_VALUEs into blocks) into
its own function.
* Name some useful types.
* Rename "ScopeToBlocks" to "ScopeToAssignBlocks", as that's what the
collection contains, blocks where assignments happen.
Differential Revision: https://reviews.llvm.org/D118454