[LangRef] Clarify alloca of zero bytes.

Let's be conservative here; it matches what we actually implemented, and
it should be rare in practice anyway.

Differential Revision: https://reviews.llvm.org/D49042



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336744 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed