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.

R=yangguo@chromium.org
BUG=v8:5400

Review-Url: https://codereview.chromium.org/2449353002
Cr-Commit-Position: refs/heads/master@{#40577}