[IPCP] Bail on extractvalue's with more than 1 index.

The replacement code only looks at the first index of the
extractvalue. If there are additional indices we'll end
up doing a bad replacement.

This only happens if the function returns a nested struct. Not
sure if clang ever generates such code. The original report came
from ispc.

Fixes PR43857

Differential Revision: https://reviews.llvm.org/D69656