Teach canonicalize pass to unique and hoist constants to the entry block. This

is a straight-forward change, but required adding missing moveBefore() methods
on operations (requiring moving some traits around to make C++ happy).  This
also fixes a constness issue with the getBlock/getFunction() methods on
Instruction, and adds a missing getFunction() method on MLFuncBuilder.

PiperOrigin-RevId: 218523905