Add call sequence start and end for __tls_get_addr

This is a fix for bug http://llvm.org/bugs/show_bug.cgi?id=25839.

For a PIC TLS variable access in a function, prologue (mflr followed by std and
stdu) gets scheduled after a tls_get_addr call. tls_get_addr messed up LR but
no one saves/restores it.

Also added a test for save/restore clobbered registers during calling __tls_get_addr.

Patch by Tim Shen

llvm-svn: 257137