Segfault due to bad inference of overlayed merge

```
using CassetteOverlay
@MethodTable SinTable;
pass = @overlaypass SinTable;
f() = Base.merge((;), Base.inferencebarrier((; a=1, b=2)))
pass(f)
```

```
[1722766] signal (4.2): Illegal instruction
in expression starting at REPL[7]:1
##SinTable#296 at ./REPL[6]:1 [inlined]
##SinTable#296 at /home/keno/.julia/dev/CassetteOverlay/src/CassetteOverlay.jl:0
```