Teach the latency scheduler some new tricks. In particular, to break ties,

keep track of a sense of "mobility", i.e. how many other nodes scheduling one
node will free up.  For something like this:

float testadd(float *X, float *Y, float *Z, float *W, float *V) {
  return (*X+*Y)*(*Z+*W)+*V;
}

For example, this makes us schedule *X then *Y, not *X then *Z.  The former
allows us to issue the add, the later only lets us issue other loads.

This turns the above code from this:

_testadd:
        lfs f0, 0(r3)
        lfs f1, 0(r6)
        lfs f2, 0(r4)
        lfs f3, 0(r5)
        fadds f0, f0, f2
        fadds f1, f3, f1
        lfs f2, 0(r7)
        fmadds f1, f0, f1, f2
        blr

into this:

_testadd:
        lfs f0, 0(r6)
        lfs f1, 0(r5)
        fadds f0, f1, f0
        lfs f1, 0(r4)
        lfs f2, 0(r3)
        fadds f1, f2, f1
        lfs f2, 0(r7)
        fmadds f1, f1, f0, f2
        blr


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26680 91177308-0d34-0410-b5e6-96231b3b80d8

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