Convert an assertion into a `Verifier` check. Bit piece expressions
must fit inside the variable, and mustn't be the entire variable.
Catching this in the verifier will help us find bugs sooner, and makes
`DIVariable::getSizeInBits()` dead code.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234698 91177308-0d34-0410-b5e6-96231b3b80d8