[x86] Hoist conditions from *every single if* in this routine to
a single early exit.

And factor the subsequent cast<> from all but one block into a single
variable.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216645 91177308-0d34-0410-b5e6-96231b3b80d8