[CallSiteSplitting] Only record conditions up to the IDom(call site).

We can stop recording conditions once we reached the immediate dominator
for the block containing the call site. Conditions in predecessors of the
that node will be the same for all paths to the call site and splitting
is not beneficial.

This patch makes CallSiteSplitting dependent on the DT anlysis. because
the immediate dominators seem to be the easiest way of finding the node
to stop at.

I had to update some exiting tests, because they were checking for
conditions that were true/false on all paths to the call site. Those
should now be handled by instcombine/ipsccp.

Reviewers: davide, junbuml

Reviewed By: junbuml

Differential Revision: https://reviews.llvm.org/D44627

llvm-svn: 346483