Recommit "[BitcodeReader] Validate OpNum, before accessing Record array."

This recommits r365750 (git commit 8b222ecf)

Original message:

   Currently invalid bitcode files can cause a crash, when OpNum exceeds
   the number of elements in Record, like in the attached bitcode file.

   The test case was generated by clusterfuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=15698

   Reviewers: t.p.northover, thegameg, jfb

   Reviewed By: jfb

   Differential Revision: https://reviews.llvm.org/D64507

   llvm-svn: 365750jkkkk

llvm-svn: 366018