[LSR] Limit the recursion for setup cost

In some circumstances we can end up with setup costs that are very complex to
compute, even though the scevs are not very complex to create. This can also
lead to setupcosts that are calculated to be exactly -1, which LSR treats as an
invalid cost. This patch puts a limit on the recursion depth for setup cost to
prevent them taking too long.

Thanks to @reames for the report and test case.

Differential Revision: https://reviews.llvm.org/D60944

llvm-svn: 358958