[InstCombine] Fold IntToPtr/PtrToInt to bitcast

The inttoptr/ptrtoint roundtrip optimization is not always correct.
We are working towards removing this optimization and adding support
to specific cases where this optimization works. This patch is the
first one on this line.

Consider the example:

    %i = ptrtoint i8* %X to i64
    %p = inttoptr i64 %i to i16*
    %cmp = icmp eq i8* %load, %p

In this specific case, the inttoptr/ptrtoint optimization is correct
as it only compares the pointer values. In this patch, we fold
inttoptr/ptrtoint to a bitcast (if src and dest types are different).

Differential Revision: https://reviews.llvm.org/D105088