[ShrinkWrapping] Handle restores on no-return paths

Shrink-wrapping uses post-dominators to find a restore point that
post-dominates all the uses of CSR / stack.

The way dominator trees are modeled in LLVM today is that unreachable
blocks are not present in a generic dominator tree, so, an unreachable node is
dominated by anything: include/llvm/Support/GenericDomTree.h:467.

Since for post-dominators, a no-return block is considered
"unreachable", calling findNearestCommonDominator on an unreachable node
A and a non-unreachable node B, will return B, which can be false. If we
find such node, we bail out since there is no good restore point
available.

rdar://problem/30186931

llvm-svn: 303130