PeepholeOptimizer: Ignore dead implicit defs

PeepholeOptimizer: Ignore dead implicit defs

Target-specific instructions may have uninteresting physreg clobbers,
for target-specific reasons. The peephole pass doesn't need to concern
itself with such defs, as long as they're implicit and marked as dead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255182 91177308-0d34-0410-b5e6-96231b3b80d8