[llvm] [DAG] ISD::matchUnaryPredicate / matchUnaryFpPredicate / matchBinaryPredicate - add DemandedElts variant (PR #183013)

[via llvm-commits]

VachanVY wrote:

@RKSimon @arsenm 

Could you please guide me on how to proceed? Either we go back to old way of switch/vector handling or have `AllowTruncation` has true because of the failing tests. I couldn't come up with anything else.

```cpp
bool SelectionDAG::isKnownToBeAPowerOfTwo(SDValue Val,
                                          const APInt &DemandedElts,
                                          bool OrZero, unsigned Depth) const {
   ... 
  // Is the constant a known power of 2 or zero?
  if (ISD::matchUnaryPredicate(Val, DemandedElts, IsPowerOfTwoOrZero,
                               /*AllowUndefs=*/false, /*AllowTruncation=*/true))
        return true;
  ...
}
```
```
  Failed Tests (3):
    LLVM :: CodeGen/RISCV/rvv/fshr-fshl-vp.ll
    LLVM-Unit :: Target/AArch64/./AArch64Tests.exe/AArch64SelectionDAGTest/KnownToBeAPowerOfTwo_Constants
    LLVM-Unit :: Target/AArch64/./AArch64Tests.exe/AArch64SelectionDAGTest/KnownToBeAPowerOfTwo_Select
``` 

As I said earlier:
> I think we need to keep AllowTruncation=true here because this call is replacing the previous explicit vector constant handling in isKnownToBeAPowerOfTwo.
Without AllowTruncation, matchUnaryPredicate rejects BUILD_VECTOR/SPLAT_VECTOR constants whose stored constant type differs from the vector scalar type, for example i32 constants used to build a v2i16. The predicate itself already does zextOrTrunc(BitWidth), so accepting those constants is intentional and matches the behavior of the removed switch/vector handling.

https://github.com/llvm/llvm-project/pull/183013