More refactoring of branch utility code to remove boilerplate. (#3448)

This is almost NFC, but it may emit slightly different IR in cases that
don't matter much. Specifically,

(block (result i32) ;; can also be unreachable
  (unreachable)
  (i32.const 1)
)

That can be finalized to have type unreachable or i32, as both are
valid. After this PR we should consistently do the same thing in all
places. (Either option would be ok - we prefer to keep the type if
there is one.)

In practice, DCE will remove all the dead code anyhow, leaving no
difference to matter. However, the IR is different without DCE, and
that may be noticeable in an unoptimized build - but it should have
no effect on behavior, just on the binary.

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