Fixes: #84018
With `-Z instrument-coverage`, coverage reporting of dead blocks
(for example, blocks dropped because a conditional branch is dropped,
based on const evaluation) is now supported.
If `instrument-coverage` is enabled, `simplify::remove_dead_blocks()`
finds all dropped coverage `Statement`s and adds their `code_region`s as
`Unreachable` coverage `Statement`s to the `START_BLOCK`, so they are
still included in the coverage map.
Check out the resulting changes in the test coverage reports in this PR.
Use `reachable_as_bitset` to reuse a bitset from the traversal rather
than allocating it seprately. Additionally check if there are any
unreachable blocks before proceeding.
Update affected ui & incremental tests to use a user declared variable
bindings instead of temporaries. The former are preserved because of
debuginfo, the latter are not.
The simplify locals implementation uses two different visitors to update
the locals use counts. The DeclMarker calculates the initial use counts.
The StatementDeclMarker updates the use counts as statements are being
removed from the block.
Replace them with a single visitor that can operate in either mode,
ensuring consistency of behaviour.
Additionally use exhaustive match to clarify what is being optimized.
No functional changes intended.
