Check for unreachable blocks in merge-return. (#1966)

Merge return assumes that the only unreachable blocks are those needed
to keep the structured cfg valid.  Even those must be essentially empty
blocks.

If this is not the case, we get unpredictable behaviour.  This commit
add a check in merge return, and emits an error if it is not the case.

Added a pass of dead branch elimination before merge return in both the
performance and size passes.  It is a precondition of merge return.

Fixes #1962.