Add operand checks for `MDLexicalBlock` and `MDLexicalBlockFile`. Like
`MDLocalVariable` and `MDLocation`, these nodes always require a scope.
There was no test bitrot to fix here (just updated the serialization
tests in test/Assembler/mdlexicalblock.ll).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233561 91177308-0d34-0410-b5e6-96231b3b80d8