[llvm-exegesis] Ignore the instructions for which InstrDesc.getSchedClass() == 0 (llvm#143840)

This allows llvm-exegesis to skip instructions that lack scheduling
information, avoiding invalid benchmarking. e.g. `InstB` in RISC-V.

Author: tclin914

llvm/test/tools/llvm-exegesis/RISCV/unsupported-opcode.test

@@ -0,0 +1,3 @@
+# RUN: llvm-exegesis -mtriple=riscv64-unknown-linux-gnu -mcpu=generic --benchmark-phase=assemble-measured-code -mode=inverse_throughput -opcode-name=InsnB 2>&1 | FileCheck %s
+
+CHECK: Unsupported opcode: No Sched Class

llvm/tools/llvm-exegesis/lib/Target.cpp

@@ -45,6 +45,8 @@ ExegesisTarget::getIgnoredOpcodeReasonOrNull(const LLVMState &State,
     return "Unsupported opcode: isBranch/isIndirectBranch";
   if (InstrDesc.isCall() || InstrDesc.isReturn())
     return "Unsupported opcode: isCall/isReturn";
+  if (InstrDesc.getSchedClass() == 0)
+    return "Unsupported opcode: No Sched Class";
   return nullptr;
 }