Fix PR5410: LiveVariables lost subreg def:

Fix PR5410: LiveVariables lost subreg def:

D0<def,dead> = ...
...
             = S0<use, kill>
S0<def>      = ...
...
D0<def>      =

The first D0 def is correctly marked dead, however, livevariables should have
added an implicit def of S0 or we end up with a use without a def.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@88690 91177308-0d34-0410-b5e6-96231b3b80d8