When simplifying an instruction that has been re-mapped, it should never
simplify to an instruction in the original function. In the edge case
where we are inlining a function into itself, the existing code led to
incorrect behavior. It would double-map an instruction: map-simplify-map.
The last map was incorrect, because the mapped instruction simplified to an instruction in the original portion of the function. An incoming value.
mapping this instruction would select the mapped version of this instruction in the newly inlined portion of the function. This wasn't correct, and would create uses not dominated by defs.
Replace the incorrect code with an assert verifying
that we never expect simplification to produce an instruction in the old
function, unless the functions are the same.