[SelectionDAG] Compute known bits and num sign bits for live out vector...

[SelectionDAG] Compute known bits and num sign bits for live out vector registers. Use it to add AssertZExt/AssertSExt in the live in basic blocks

Summary:
We already support this for scalars, but it was explicitly disabled for vectors. In the updated test cases this allows us to see the upper bits are zero to use less multiply instructions to emulate a 64 bit multiply.

This should help with this ispc issue that a coworker pointed me to https://github.com/ispc/ispc/issues/1362

Reviewers: spatel, efriedma, RKSimon, arsenm

Reviewed By: spatel

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D54725

llvm-svn: 347287