Add an explicit insert point argument to SplitBlockAndInsertIfThen.

Currently SplitBlockAndInsertIfThen requires that branch condition is an
Instruction itself, which is very inconvenient, because it is sometimes an
Operator, or even a Constant.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197677 91177308-0d34-0410-b5e6-96231b3b80d8