If my misocompile, you mean that it fails the test that a "C expression `a += 1` can be trusted to increment a numeric value", then it is trivial: https://godbolt.org/z/G5dP9dM5q
The (implied) claim is that the C standard has enough sources of undefined behavior that even a simple integer addition can't be relied upon to actually perform integer addition.
But the sources of undefined behavior for integer addition in C are well-known and very clear, and any instruction set that isn't an insane science project is going to have an instruction to add integers.
Thus my comment. Show me a C compiler that takes that code and miscompiles it. I don't care if it returns a constant, spits out an infinite loop, jumps to 0x0000, calls malloc, whatever. Show me a C compiler that takes those four lines of C code and emits something other than an integer addition instruction.