[ValueTracking] Add assertions that the starting Depth in...

[ValueTracking] Add assertions that the starting Depth in isKnownToBeAPowerOfTwo and ComputeNumSignBitsImpl is not above MaxDepth

The function does an equality check later to terminate the recursion, but that won't work if its starts out too high. Similar assert already exists in computeKnownBits.

llvm-svn: 311400