IR: Take advantage of -verify checks for MDExpression

Now that we check `MDExpression` during `-verify` (r232299), make
the `DIExpression` wrapper more strict:

  - remove redundant checks in `DebugInfoVerifier`,
  - overload `get()` to `cast_or_null<MDExpression>` (superseding
    `getRaw()`),
  - stop checking for null in any accessor, and
  - remove `DIExpression::Verify()` entirely in favour of
    `MDExpression::isValid()`.

There is still some logic in this class, mostly to do with high-level
iterators; I'll defer cleaning up those until the rest of the wrappers
are similarly strict.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232412 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Verifier.cpp

@@ -3078,15 +3078,11 @@ void DebugInfoVerifier::processCallInst(DebugInfoFinder &Finder,
       case Intrinsic::dbg_declare: {
         auto *DDI = cast<DbgDeclareInst>(&CI);
         Finder.processDeclare(*M, DDI);
-        if (auto E = DDI->getExpression())
-          Assert(DIExpression(E).Verify(), "DIExpression does not Verify!", E);
         break;
       }
       case Intrinsic::dbg_value: {
         auto *DVI = cast<DbgValueInst>(&CI);
         Finder.processValue(*M, DVI);
-        if (auto E = DVI->getExpression())
-          Assert(DIExpression(E).Verify(), "DIExpression does not Verify!", E);
         break;
       }
       default: