Revert "[SLP]Do not require external uses for roots and single use for other...

Revert "[SLP]Do not require external uses for roots and single use for other instructions in computeMinimumValueSizes. (#72679)"

This reverts commit 408dce82.

This triggered failed asserts with code like this:

    char a[];
    short *b;
    int c, d, e, f;
    void g() {
      char *h;
      for (;;) {
        for (; f; ++f) {
          h[f] = b[0] * a[e] + b[c] * a[1] >> 7;
          ++b;
        }
        h += d;
      }
    }

Compiled like this:

    $ clang -target x86_64-linux-gnu -c repro.c -O2
    clang: ../lib/IR/Instructions.cpp:3335: static llvm::CastInst* llvm::CastInst::Create(llvm::Instruction::CastOps, llvm::Value*, llvm::Type*, const llvm::Twine&, llvm::Instruction*): Assertion `castIsValid(op, S, Ty) && "Invalid cast!"' failed.