[llvm] [RISCV] Add TSFlag for reading past VL behaviour. NFCI (PR #149704)

[Luke Lau via llvm-commits]

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@@ -1315,7 +1287,8 @@ RISCVVLOptimizer::getMinimumVLForUser(const MachineOperand &UserOp) const {
     return std::nullopt;
   }
 
-  if (mayReadPastVL(UserMI)) {
+  if (RISCVII::readsPastVL(
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lukel97 wrote:

I guess that ended up being a more contentious change than expected. Maybe instead of adding a TSFlag we could make it a predicate in RISCVInstrPredicates.td? I think that will still require writing down a lot of different opcodes though. 

https://github.com/llvm/llvm-project/pull/149704