ssr: register merging not working when value used outside of the basic block

in case the use of an ssr_pop(0) is in a different BB, the register merging just eliminates the `pop` but doesn't replace the actual use of e.g. `ft0`. 

```
bb.0:
  %0:fpr = PeudoPop
bb.1:
  # some use of %0
```

this then leads to a crash of the compiler.

It can be mitigated with `-mllvm -ssr-noregmerge` but that is not the idea of SSRs