[LoopUnroll] Ignore ephemeral values when checking full unroll profitability.

Summary:
Before this patch call graph is like this in the LoopUnrollPass:

  tryToUnrollLoop
    ApproximateLoopSize
      collectEphemeralValues
      /* Use collected ephemeral values */
    computeUnrollCount
      analyzeLoopUnrollCost
        /* Bail out from the analysis if loop contains CallInst */

This patch moves collection of the ephemeral values to the tryToUnrollLoop
function and passes the collected values into both ApproximateLoopsize (as
before) and additionally starts using them in analyzeLoopUnrollCost:

  tryToUnrollLoop
    collectEphemeralValues
    ApproximateLoopSize(EphValues)
      /* Use EphValues */
    computeUnrollCount(EphValues)
      analyzeLoopUnrollCost(EphValues)
        /* Ignore ephemeral values - they don't contribute to the final cost */
        /* Bail out from the analysis if loop contains CallInst */

Reviewers: mzolotukhin, evstupac, sanjoy

Reviewed By: evstupac

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43931

llvm-svn: 327617