Allow PeepholeOptimizer to fold a few more cases

Allow PeepholeOptimizer to fold a few more cases

The condition for clearing the folding candidate list was clamped together
with the "uninteresting instruction" condition. This is too conservative,
e.g. we don't need to clear the list when encountering an IMPLICIT_DEF.

Differential Revision: http://reviews.llvm.org/D11591

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244577 91177308-0d34-0410-b5e6-96231b3b80d8