Revert "Reland "[TargetLowering] Teach DemandedBits about VSCALE""

This reverts commit 3010f603.

This change introduced undefined behaviour (reported at
https://reviews.llvm.org/D138508#inline-1352840).  Additionally, it
appears to be responsible for a mis-compilation on RISCV64 with the
vector extension (https://github.com/llvm/llvm-project/issues/59594).
The commit message indicates that this is meant to be ARM64 specific
though is a generic selection change.