[DAGCombine] (float)((int) f) --> ftrunc (PR36617)

This was originally committed at rL328921 and reverted at rL329920 to
investigate failures in Chrome. This time I've added to the ReleaseNotes
to warn users of the potential of exposing UB and let me repeat that
here for more exposure:

  Optimization of floating-point casts is improved. This may cause surprising
  results for code that is relying on undefined behavior. Code sanitizers can
  be used to detect affected patterns such as this:

    int main() {
      float x = 4294967296.0f;
      x = (float)((int)x);
      printf("junk in the ftrunc: %f\n", x);
      return 0;
    }

    $ clang -O1 ftrunc.c -fsanitize=undefined ; ./a.out
    ftrunc.c:5:15: runtime error: 4.29497e+09 is outside the range of 
                   representable values of type 'int'
    junk in the ftrunc: 0.000000


Original commit message:

fptosi / fptoui round towards zero, and that's the same behavior as ISD::FTRUNC,
so replace a pair of casts with the equivalent node. We don't have to account for
special cases (NaN, INF) because out-of-range casts are undefined.

Differential Revision: https://reviews.llvm.org/D44909

llvm-svn: 330437