[InstCombine] try to convert vector insert+extract to trunc

This transform is requested for the backend in:
https://bugs.llvm.org/show_bug.cgi?id=39016
...but I figured it was worth doing in IR too, and it's probably 
easier to implement here, so that's this patch.

In the simplest case, we are just truncating a scalar value. If the 
extract index doesn't correspond to the LSBs of the scalar, then we 
have to shift-right before the truncate. Endian-ness makes this tricky, 
but hopefully the ASCII-art helps visualize the transform.

Differential Revision: https://reviews.llvm.org/D52439

llvm-svn: 343407