[DAG] Replace getValid*ShiftAmountConstant helpers with getValid*ShiftAmount...

[DAG] Replace getValid*ShiftAmountConstant helpers with getValid*ShiftAmount helpers to support KnownBits analysis (#93182)

The getValidShiftAmountConstant/getValidMinimumShiftAmountConstant/getValidMaximumShiftAmountConstant helpers only worked with constant shift amounts, which could be problematic after type legalization (e.g. v2i64 might be partially scalarized or split into v4i32 on some targets such as 32-bit x86, Thumb2 MVE).

This patch proposes we generalize these helpers to work with ConstantRange+KnownBits if a scalar/buildvector constant isn't available.

Most restrictions are the same - the helper fails if any shift amount is out of bounds, getValidShiftConstant must be a specific constant uniform etc.

However, getValidMinimumShiftAmount/getValidMaximumShiftAmount now can return bounds values that aren't values in the actual data, as they are based off the common KnownBits of every vector element.

This addresses feedback on #92096