[VirtRegRewriter] Insert missing killed flags when tracking subregister liveness

VirtRegRewriter may sometimes fail to correctly apply the kill flag where necessary,
which causes unecessary code gen on PowerPC. This patch fixes the way masks for
defined lanes are computed and the way mask for used lanes is computed.

Contact albion.fung@ibm.com instead of author for problems related to this commit.

Differential Revision: https://reviews.llvm.org/D92405