[PowerPC] fix potential verification error on __tls_get_addr

[PowerPC] fix potential verification error on __tls_get_addr

This patch fixes a verification error with -verify-machineinstrs while expanding __tls_get_addr by not creating ADJCALLSTACKUP and ADJCALLSTACKDOWN if there is another ADJCALLSTACKUP in this basic block since nesting ADJCALLSTACKUP/ADJCALLSTACKDOWN is not allowed.

Here, ADJCALLSTACKUP and ADJCALLSTACKDOWN are created as a fence for instruction scheduling to avoid _tls_get_addr is scheduled before mflr in the prologue (https://bugs.llvm.org//show_bug.cgi?id=25839). So if another ADJCALLSTACKUP exists before _tls_get_addr, we do not need to create a new ADJCALLSTACKUP.

Differential Revision: https://reviews.llvm.org/D34347

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306678 91177308-0d34-0410-b5e6-96231b3b80d8