Remove bogus "fast path" in unroller branch update

Remove bogus "fast path" in unroller branch update

BasicBlocks whose `bbJumpKind` doesn't make use of `bbJumpDest` may have
arbitrary garbage in their `bbJumpDest` field, so remove the code from
loop unrolling that was expecting such blocks to have null `bbJumpDest`s,
and instead always defer to the `optCopyBlkDest`/`optRedirectBlock`
helpers that check the jump kind before checking the jump dest.

Fixes #8231.