Revert r343407 "[InstCombine] try to convert vector insert+extract to trunc"

This caused Chromium builds to fail with "Illegal Trunc" assertion.
See https://crbug.com/890723 for repro.

> 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: 343458