Are you claiming that there must be such transforms or only that it is likely that bugs exist which isn't interesting.
To assert that something specific always happens under UB is counter to the definition of UB. Fil-C carefully defines away UB for some operations, but to make full guarantee of safety, even by their definition and modulo bugs, I believe it necessary to fully remove UB.
Fil-C gets to look at some code which has UB if variable z is 9 and go "OK, lets check whether z is 9, and if so...". The result, of course, is much, much slower than executables from a typical modern C compiler, but since "It always does X" is in fact a permitted implementation of "Undefined Behaviour" this is a compliant C implementation, at least in most observable respects.