[llvm] [SelectionDAG][WebAssembly] Tidy up around endianess and isConstantSplat (PR #68212)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@bjope LGTM, but it scares me that the isBigEndian argument still defaults to little endian. I think this to minimize changes when it was pulled from x86, but something so generic really shouldn't be making this assumption any more.

https://github.com/llvm/llvm-project/pull/68212