Fix the segfault reported in PR 11990.

The sefault occurs due to an infinite loop when the verifier tries to
determine the size of a type of the form "%rt = type { %rt }" while
checking an alloca of the type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196626 91177308-0d34-0410-b5e6-96231b3b80d8