[Bitcode reader] Fix a few assertions when reading invalid files

Summary:
When creating {insert,extract}value instructions from a BitcodeReader, we
weren't verifying the fields were valid.

Bugs found with afl-fuzz

Reviewers: rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7325

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229345 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed