[AArch64][SME]: Generate streaming-compatible code for bit counting/select

To generate code compatible to streaming mode:
 - enable custom-lowering ISD::CTLZ and ISD::CTPOP.
 - disable combining OR into BSL.

- Testing files:
 - bit-counting.ll
 - bitselect.ll

Reviewed By: david-arm, sdesmalen

Differential Revision: https://reviews.llvm.org/D138682