[CFIInserter] Turn a reachable llvm_unreachable into a report_fatal_error. (#168777)
This prevents it from being optimized out in non-asserts builds.
Update X86 test to remove REQUIRES: asserts and check for LLVM ERROR.
Add FileCheck to RISC-V test and remove UNSUPPORTED.
This is the more complete fix for #168772 and #168525.