[CodeGen] Prevent nullptr deref in genAlternativeCodeSequence

[fdeazeve]

Summary

A pointer dereference was added (D141302) above an assert that checks
whether the pointer is null. This commit moves the assert above the
dereference and transforms it into an llvm_unreachable to better express
the intent that certain switch cases should never be reached.

[fdeazeve]

Update commit message

[fhahn]

LGTM, thanks!