Revert "[SLP] Consider alternatives for cost of select instructions."

This reverts commit 19225704.

This appears to cause a crash in the following example

 a, b, c;
 l() {
   int e = a, f = l, g, h, i, j;
   float *d = c, *k = b;
   for (;;)
     for (; g < f; g++) {
       k[h] = d[i];
       k[h - 1] = d[j];
       h += e << 1;
       i += e;
     }
 }

 clang -cc1 -triple i386-unknown-linux-gnu -emit-obj -target-cpu pentium-m -O1 -vectorize-loops -vectorize-slp reduced.c

 llvm::Type *llvm::Type::getWithNewBitWidth(unsigned int) const: Assertion `isIntOrIntVectorTy() && "Original type expected to be a vector of integers or a scalar integer."' failed.