[TableGen][NFC] convert TreePatternNode pointers to references (#81134)

Almost all uses of `*TreePatternNode` expect it to be non-null. There
was the occasional check that it wasn't, which I have removed. Making
them references makes it clear that they exist.

This was attempted in 2018 (1b465767)
for `TreePatternNode::getChild()` but that was reverted.