[DAGCombiner] restrict (float)((int) f) --> ftrunc with no-signed-zeros

As noted in the D44909 review, the transform from (fptosi+sitofp) to ftrunc 
can produce -0.0 where the original code does not:

#include <stdio.h>
  
int main(int argc) {
  float x;
  x = -0.8 * argc;
  printf("%f\n", (float)((int)x));
  return 0;
}

$ clang -O0 -mavx fp.c ; ./a.out 
0.000000
$ clang -O1 -mavx fp.c ; ./a.out 
-0.000000

Ideally, we'd use IR/node flags to predicate the transform, but the IR parser 
doesn't currently allow fast-math-flags on the cast instructions. So for now, 
just use the function attribute that corresponds to clang's "-fno-signed-zeros" 
option.

Differential Revision: https://reviews.llvm.org/D48085

llvm-svn: 335761