Generate parameter pattern-match IR from pattern IR

[geoffromer]

Also propagate the pattern IR along with the pattern-match IR, and use it where appropriate.

Strictly speaking, some parts of the pattern-match IR are allocated eagerly, while traversing the pattern's parse tree, but they still aren't actually emitted until we traverse the associated pattern insts.

[jonmeow]

I don't expect that we both need to review this, I'm going to let zygoloid handle it if he wants. :)

[geoffromer]

Looks good, hopefully the merge isn't too painful.

One of the least painful merges I've done recently, in fact :-)