Revert "[SLP]Improve minbitwidth analysis."

This reverts commit 2bd369b4.

That commit triggered failed assertions:
$ cat repro.c
short *a;
int b;
void h() {
  short *c = a;
  b = 0;
  for (; b < 4; b++) {
    unsigned d = a[b] + a[b + 4 * 2], e = a[b] - a[b + 4 * 2],
             f = (a[b + 4] >> 1) - a[b + 4 * 3],
             g = a[b + 4] + (a[b + 4 * 3] >> 1);
    c[b] = g;
    c[b + 4] = e + f;
    c[b + 4 * 2] = e - f;
    c[b + 4 * 3] = d - g;
  }
}
$ clang -target aarch64-linux-gnu -c -O2 repro.c
clang: ../lib/Transforms/Vectorize/SLPVectorizer.cpp:12503: llvm::Value* llvm::slpvectorizer::BoUpSLP::vectorizeTree(llvm::slpvectorizer::BoUpSLP::TreeEntry*, bool): Assertion `(MinBWs.contains(getOperandEntry(E, 0)) || MinBWs.contains(getOperandEntry(E, 1))) && "Expected item in MinBWs."' failed.