Improve placement of IPA tie bars

Consider improving the placement of the IPA tie bars in Source Code Pro:


Here’s a [PDF of Source Code Pro version 2.030](https://github.com/adobe-fonts/source-code-pro/files/668592/SourceCodePro-IPA.pdf) rendering an [IPA test page](https://github.com/googlei18n/noto-source/blob/master/test/und-fonipa.html) which I originally made for Noto, another open-source font. Admittedly my test cases are quite incomplete, but feel free to use them nevertheless; also, don’t hesitate to send pull requests for improving the tests :-)