[Hexagon] Add implicit uses even when untied explicit uses are present

An explicit untied use is not sufficient to maintain liveness of a
register redefined in a predicated instruction. For example
  %1 = COPY %0
  ...
  %1 = A2_paddif %2, %1, 1
could become
  $r1 = COPY $r0
  ...
  $r1 = A2_paddif $p0, $r1, 1
and later
  $r1 = COPY $r0                ;; this is not really dead!
  ...
  $r1 = A2_paddif $p0, $r0, 1

llvm-svn: 336662