[TargetLowering][AArch64] Teach DemandedBits about SVE count intrinsics

This allows DemandedBits to see that the SVE count intrinsics (CNTB,
CNTH, CNTW, CNTD) sans multiplier will only ever produce small
positive integers. The maximum value you could get here is 256, which
is CNTB on a machine with a 2048bit vector size (the maximum for SVE).

Using this various redundant operations (zexts, sexts, ands, ors, etc)
can be eliminated.

Differential Revision: https://reviews.llvm.org/D138424