[RISCV][VLOPT] Move mayReadPastVL check into getMinimumVLForUser. NFC (#127972)

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

@@ -1272,6 +1272,11 @@ RISCVVLOptimizer::getMinimumVLForUser(const MachineOperand &UserOp) const {
     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.
@@ -1335,11 +1340,6 @@ RISCVVLOptimizer::checkUsers(const MachineInstr &MI) const {
       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;