[SME2/SVE2p1] Extend llvm.aarch64.sve.convert.to/from.svbool to accept target("aarch64.svcount")

The convert intrinsics can be used to implement existing operations on svcount_t
when the actual bits/content of the predicate register doesn't matter (such
as PSEL, which copies the full contents of the first source register to the
destination register).

Reviewed By: CarolineConcatto, david-arm

Differential Revision: https://reviews.llvm.org/D150959