Cranelift: Refactor legalization such that traversal is separate from legalizing a particular instruction

Notably, make sure that we aren't using the same `FuncCursor` for traversal and
modification, as that makes it hard to follow when the traversal is being moved
forward and backed up.

Additionally, this makes it so that when we legalize an instruction, we only
back up the cursor to continue legalization until we reach a fixed-point when
said legalization resulted in something that actually requires further
legalization. This should be mildly more efficient, since we can avoid an
unnecessary loop iteration and its `match` on the just-legalized instruction
data and opcode.

Author: fitzgen