LowLevelType: replace a reachable llvm_unreachable with assert

assert is more appropriate here and fixes
`runtime error: execution reached an unreachable program point` in a
-DLLVM_USE_SANITIZER=Undefined build (-fno-sanitize-recover=all causes
llc to exit instead of crash (report_fatal_error)) when testing
MachineVerifier/test_g_assert_[sz]ext.mir.

Author: MaskRay

llvm/include/llvm/CodeGen/LowLevelType.h

@@ -238,10 +238,9 @@ class LLT {
         return getFieldValue(VectorSizeFieldInfo);
       else
         return getFieldValue(PointerVectorSizeFieldInfo);
-    } else if (IsPointer)
-      return getFieldValue(PointerSizeFieldInfo);
-    else
-      llvm_unreachable("unexpected LLT");
+    }
+    assert(IsPointer && "unexpected LLT");
+    return getFieldValue(PointerSizeFieldInfo);
   }
 
   constexpr unsigned getAddressSpace() const {