give instcombine a custom IRBuilder that adds new instructions to the

give instcombine a custom IRBuilder that adds new instructions to the
workslist and is set to insert new instructions before the current one.
Convert a bunch of stuff that used to call InsertNewInstBefore over to
use it, greatly simplifying code and making it more natural.

There is still a lot more to go, but this is a good start.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@80492 91177308-0d34-0410-b5e6-96231b3b80d8