[TLS] move setting tls_guard in tls_init.

[TLS] move setting tls_guard in tls_init.

We used to emit the store prior to branch in the entry block. To make it more
efficient, this commit moves it to the init block. We still mark as initialized
before initializing anything else.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252777 91177308-0d34-0410-b5e6-96231b3b80d8