fold trivial token factor nodes. This allows us to compile

fold trivial token factor nodes.  This allows us to compile
test/CodeGen/X86/fp-stack-ret.ll into:

        movl 4(%esp), %eax
        fldl (%eax)
        ret

instead of:

        subl $12, %esp
        movl 16(%esp), %eax
        movsd (%eax), %xmm0
        movsd %xmm0, (%esp)
        fldl (%esp)
        addl $12, %esp
        ret

by eliminating a token factor that blocked a check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34584 91177308-0d34-0410-b5e6-96231b3b80d8