[AArch64][SME]: Generate streaming-compatible code for fp_to_int and int_to_fp.

1-To generate code compatible to streaming mode:
  - enable custom lowering fp_to_int, int_to_fp, fp_round, fmul, trunc.
  - disable fp_to_int combining into invalid NEON intrinsic.
2-Add testing files:
  fp_to_int.ll
  fp-convert.ll
  int_to_fp.ll
  trunc.ll

Differential Revision: https://reviews.llvm.org/D138281