[RISCV][NFC] Improve encoding/decoding tests for Zbb instructions

Currently `llvm/test/MC/RISCV/rv64zbb-valid.s` doesn't cover all
the instructions e.g. `maxu` and `llvm/test/MC/RISCV/rv32zbb-valid.s`
can have rv64 run lines, similar to what Zba instruction tests have.

This patch does the following.
- Add rv64 run lines in `llvm/test/MC/RISCV/rv32zbb-valid.s`
- Keep only rv64 specific instructions in `llvm/test/MC/RISCV/rv64zbb-valid.s`
- Move rv32 instructions, with different encodings from rv64, into
  `llvm/test/MC/RISCV/rv32zbb-only-valid.s`

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D152680