[ValueTracking] Look through casts when determining non-nullness

[ValueTracking] Look through casts when determining non-nullness

Bitcast and certain Ptr2Int/Int2Ptr instructions will not alter the
value of their operand and can therefore be looked through when we
determine non-nullness.

Differential Revision: https://reviews.llvm.org/D54956

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352293 91177308-0d34-0410-b5e6-96231b3b80d8