[OpenCL] Restore r338899 (reverted in r338904), fixing stack-use-after-return

Always emit alloca in entry block for enqueue_kernel builtin.

Ensures the statically sized alloca is not converted to DYNAMIC_STACKALLOC
later because it is not in the entry block.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@339150 91177308-0d34-0410-b5e6-96231b3b80d8