[SelectionDAG] Adjust placement of blank lines in debug output. NFC (#70969)

Also add a blank line to the beginning of every dump of the SelectionDAG
to visually separate it from whatever the last message was.

Add a blank line before "Legalizing node:" in the type legalizer. Remove
the blank line after things like "Promote integer result:".

This keeps all of the messages for the legalization of a single node
grouped together with no blank lines in the middle. This is consistent
with the operation legalizers.