[DSE] Make isRemovable() for calls more robust (NFCI)

[nikic]

Summary

We can only drop calls if they have an analyzable write, the return value is not used, they don't throw and they don't diverge. The last two conditions were previously not checked, because all the libcalls with analyzable writes already happened to satisfy those conditions anyway. This may not be true for generalizations (with D115904 in mind).

No test changes because the necessary attributes are already inferred for supported libcalls.

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
     case Intrinsic::memcpy_element_unordered_atomic:
     case Intrinsic::memmove_element_unordered_atomic:
     case Intrinsic::memset_element_unordered_atomic:
     case Intrinsic::masked_store:
       return true;
     }
   }
 
-  // note: only get here for calls with analyzable writes - i.e. libcalls
+  // note: only get here for calls with analyzable writes.
   if (auto *CB = dyn_cast<CallBase>(I))
-    return CB->use_empty();
+    return CB->use_empty() && CB->willReturn() && CB->doesNotThrow();
 
   return false;
 }
 
 /// Returns true if the end of this instruction can be safely shortened in
 /// length.
 static bool isShortenableAtTheEnd(Instruction *I) {
   // Don't shorten stores for now