microsoft/compiler: improve phi-handling

The PHI value handling in the DXIL back-end had an artificial limit of 127 incoming values per PHI instruction. There's no guarantee for this limit, but the value was so high that we never encountered larger shaders.

However, this is neither robust nor memory efficient. So let's lift this limitation.

This leads us to a new, similar limitation, but at 128 instead of 127. Why the two different limitations? Beats me. I wrote the code, so all I can say is that neither of these should have been left in place. So we fix both.