[llvm-extract] Fix basic block extraction by delaying search until the function is materialized

Summary: When I try to extract a basic block using llvm-extract, it erroneously reports that the named basic block doesn't exist. After digging into the code, it appears to be iterating over an unmaterialized function, which will fail to match any basic blocks.

Reviewers: volkan, qcolombet

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80346