[llvm] [AArch64][SVE] Implement demanded bits for @llvm.aarch64.sve.cntp (PR #168714)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm commented:

Perhaps the original code is backward in asking for the element size?

I wonder if it should be more literal and ask for the maximum result as an element count, or maximum unscaled result as an unsigned (not really sure which is better). Then have the caller multiply the result by the maximum value of vscale (i.e. (MaxSVEVectorSizeInBits or SVEMaxBitsPerVector) / 128)).

What do you think?

https://github.com/llvm/llvm-project/pull/168714