[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