[SVE][CodeGen] Add patterns for ADD/SUB + element count

This patch adds patterns to match the following with INC/DEC:
 - @llvm.aarch64.sve.cnt[b|h|w|d] intrinsics + ADD/SUB
 - vscale + ADD/SUB

For some implementations of SVE, INC/DEC VL is not as cheap as ADD/SUB and
so this behaviour is guarded by the "use-scalar-inc-vl" feature flag, which for SVE
is off by default. There are no known issues with SVE2, so this feature is
enabled by default when targeting SVE2.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D111441