port df981a9f (r40577)

  original commit message:
  The meaning of the HValue::kAllowUndefinedAsNaN is actually ToNumber
  conversion (except for the uses in HBranch and HCompareHoleAndBranch,
  which were confusing and useless anyways), so fix the naming to match
  that.

  Also properly integrate the handling of this flag with the existing
  truncation analysis that is run as part of the representation changes
  phase (i.e. where we already deal with truncating to int32 and smi).

  This is done in preparation of allowing Crankshaft to handle any kind
  of Oddball in the ToNumber truncation, instead of just undefined for
  truncation ToNumber and undefined or boolean for ToInt32. It also helps
  to make Crankshaft somewhat more compatible with the (saner)
  implementation in TurboFan.

BUG=

Review-Url: https://codereview.chromium.org/2456503003
Cr-Commit-Position: refs/heads/master@{#40607}