Rollup merge of #124064 - Zalathar:otherwise-block, r=Nadrieril

Move confusing comment about otherwise blocks in `lower_match_tree`

This comment was historically inside a block guarded by `if let Some(otherwise_block) = otherwise`.

When #120978 made the “otherwise block” non-optional, it also flattened that region of code. Doing so left this comment awkwardly stranded above an unrelated line of code, without its original context.

We can restore that context by moving it above the declaration of `otherwise`.

r? ``@Nadrieril``

Author: matthiaskrgr