[RISCV] Use bseti for 2048 in RISCVMatInt when Zbs is enabled.

2048 requires an LUI and ADDI instruction due to ADDI using a
signed immediate. It can also be done with C.LI+C.SLLI for better
code size.

With Zbs we can use a single BSETI to have an instruction.

Reorder the checks so that BSETI is checked first, with an extra
qualification to prefer a single LUI or ADDI when possible. I'm
continuing to think about other ways to structure this code, but
this works for now.

Fixes PR59362.