Also add <imp-use,kill> flags for redefined super-registers.

For example:

  %vreg10:dsub_0<def,undef> = COPY %vreg1
  %vreg10:dsub_1<def> = COPY %vreg2

is rewritten as:

  %D2<def> = COPY %D0, %Q1<imp-def>
  %D3<def> = COPY %D1, %Q1<imp-use,kill>, %Q1<imp-def>

The first COPY doesn't care about the previous value of %Q1, so it
doesn't read that register.

The second COPY is a partial redefinition of %Q1, so it implicitly kills
and redefines that register.

This makes it possible to recognize instructions that can harmlessly
clobber the full super-register.  The write and don't read the
super-register.

llvm-svn: 141139