Revert [BitcodeReader] Validate OpNum, before accessing Record array.

This reverts r365750 (git commit 8b222ecf)

llvm-dis runs out of memory while opening invalid-fcmp-opnum.bc on
llvm-hexagon-elf, probably because the bitcode file contains other
suspicious values.

http://lab.llvm.org:8011/builders/llvm-hexagon-elf/builds/21949

llvm-svn: 365757