X86 cost model: Model cost for uitofp and sitofp on SSE2

The costs are overfitted so that I can still use the legalization factor.

For example the following kernel has about half the throughput vectorized than
unvectorized when compiled with SSE2. Before this patch we would vectorize it.

unsigned short A[1024];
double B[1024];
void f() {
  int i;
  for (i = 0; i < 1024; ++i) {
    B[i] = (double) A[i];
  }
}

radar://13599001

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179033 91177308-0d34-0410-b5e6-96231b3b80d8
Rewrite test/Verifier tests to use FileCheck instead of grep


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179036 91177308-0d34-0410-b5e6-96231b3b80d8
Update documentation.

First feature is not CPU subtype anymore since r134127

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179038 91177308-0d34-0410-b5e6-96231b3b80d8