[ValueTracking] Honour recursion limit.

The recently improved support for `icmp` in ValueTracking
(r307304) exposes the fact that `isImplied` condition doesn't
really bail out if we hit the recursion limit (and calls
`computeKnownBits` which increases the depth and asserts).

Differential Revision:  https://reviews.llvm.org/D36512

llvm-svn: 310481