[CodeGen] Teach the peephole optimizer to remember (and exploit) all folding
opportunities in the current basic block, rather than just the last one seen.

<rdar://problem/16478629>



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205481 91177308-0d34-0410-b5e6-96231b3b80d8