Resolve conflicts involving dead vector lanes for -new-coalescer.

A common coalescing conflict in vector code is lane insertion:

  %dst = FOO
  %src = BAR
  %dst:ssub0 = COPY %src

The live range of %src interferes with the ssub0 lane of %dst, but that
lane is never read after %src would have clobbered it. That makes it
safe to merge the live ranges and eliminate the COPY:

  %dst = FOO
  %dst:ssub0 = BAR

This patch teaches the new coalescer to resolve conflicts where dead
vector lanes would be clobbered, at least as long as the clobbered
vector lanes don't escape the basic block.

llvm-svn: 164250