Fixes PR23804: assertion failure in emitPrologue in the case of a
function with an empty frame and a dynamic alloca that needs stack
realignment. This is a typical case for AddressSanitizer.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241943 91177308-0d34-0410-b5e6-96231b3b80d8