Clang for RISC-V 32/64 bits crashes on __c11_atomic_*_fence() with variable memory order

Nobody has claimed this yet.

Hi there,

The following piece of code:

```
int main(int argc, char **argv) {
__c11_atomic_signal_fence(argc);
}
```

causes Clang r318692 with today's patches to crash as follows:

```
fatal error: error in backend: Cannot select: 0x8066806e8: ch = AtomicFence 0x80661cc40, Constant:i64<7>,
Constant:i64<0>
0x806680548: i64 = Constant<7>
0x806680068: i64 = Constant<0>
In function: main
clang-6.0: error: clang frontend command failed with exit code 70 (use -v to see invocation)
clang version 6.0.0 (trunk 318692)
Target: riscv64-unknown-cloudabi
Thread model: posix
InstalledDir: /usr/local/bin
clang-6.0: note: diagnostic msg: PLEASE submit a bug report to http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and associated run script.
clang-6.0: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang-6.0: note: diagnostic msg: /tmp/bla-aa1f6f.c
clang-6.0: note: diagnostic msg: /tmp/bla-aa1f6f.sh
clang-6.0: note: diagnostic msg:

********************
```

Best regards,
Ed

Reproduce the crash by compiling the shown C snippet with Clang r318692 for the riscv64-unknown-cloudabi target. Start from the __c11_atomic_signal_fence call and the backend's AtomicFence selection failure; done means this variable-memory-order fence no longer causes a compiler crash.

Written by the indexing model from the issue text.