[llvm] [RISCV] Use a switch in RISCVInstrInfo::verifyInstruction. NFC (PR #170961)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lldb-arm-ubuntu` running on `linaro-lldb-arm-ubuntu` while building `llvm` at step 6 "test".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/18/builds/23120

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test) failure: build (failure)

```

</details>

https://github.com/llvm/llvm-project/pull/170961