[RISCV] Verify that policy operands only exist on instructions with tied passthru operands

[reames]

Summary

This is a non-trivial property relied upon by D135396. I wrote this to convince myself it was true.

[craig.topper]

LGTM. Thanks.

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

if (RISCVII::hasVecPolicyOp(TSFlags)) {
     if (Policy > (RISCVII::TAIL_AGNOSTIC | RISCVII::MASK_AGNOSTIC)) {
       ErrInfo = "Invalid Policy Value";
       return false;
     }
     if (!RISCVII::hasVLOp(TSFlags)) {
       ErrInfo = "policy operand w/o VL operand?";
       return false;
     }
+
+    // VecPolicy operands can only exist on instructions with passthru/merge
+    // arguments. Note that not all arguments with passthru have vec policy
+    // operands- some instructions have implicit policies.
+    unsigned UseOpIdx;
+    if (!MI.isRegTiedToUseOperand(0, &UseOpIdx)) {
+      ErrInfo = "policy operand w/o tied operand?";
+      return false;
+    }
   }
 
   return true;
 }
 
 // Return true if get the base operand, byte offset of an instruction and the
 // memory width. Width is the size of memory that is being loaded/stored.
 bool RISCVInstrInfo::getMemOperandWithOffsetWidth(