RISC-V: C2: Use single-bit instructions from the Zbs extension

The single-bit instructions from the Zbs extension provide a mechanism to set, clear, invert, or extract a single bit in a register. The bit is specified by its index.

Especially, the single-bit extract (immediate) instruction 'bexti rd, rs1, shamt' performs this operation:

```
  let index = shamt & (XLEN - 1);
  X(rd) = (X(rs1) >> index) & 1;
```

This instruction could be used to match for following sub-graph in C2 where integer immediate 'mask' is power of 2:
'''
   Set dst (Conv2B (AndI src mask))
'''