[RISCV][VLOPT] Move mayReadPastVL check into getMinimumVLForUser. NFC

checkUsers currently does two things, a) work out the minimum VL read by every user and b) check that the operand info of the MI and users match.

getMinimumVLForUser handles most of a), with the exception of the check for instructions that read past VL e.g. vrgather which is still in checkUsers.

This moves it into getMinimumVLForUser to keep all that logic in one place and simplifies an upcoming patch.

Author: lukel97

llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

@@ -1271,6 +1271,11 @@ RISCVVLOptimizer::getMinimumVLForUser(MachineOperand &UserOp) {
     return std::nullopt;
   }
 
+  if (mayReadPastVL(UserMI)) {
+    LLVM_DEBUG(dbgs() << "    Abort because used by unsafe instruction\n");
+    return std::nullopt;
+  }
+
   unsigned VLOpNum = RISCVII::getVLOpNum(Desc);
   const MachineOperand &VLOp = UserMI.getOperand(VLOpNum);
   // Looking for an immediate or a register VL that isn't X0.
@@ -1333,11 +1338,6 @@ std::optional<MachineOperand> RISCVVLOptimizer::checkUsers(MachineInstr &MI) {
       continue;
     }
 
-    if (mayReadPastVL(UserMI)) {
-      LLVM_DEBUG(dbgs() << "    Abort because used by unsafe instruction\n");
-      return std::nullopt;
-    }
-
     auto VLOp = getMinimumVLForUser(UserOp);
     if (!VLOp)
       return std::nullopt;