[RISCV] Remove support for pre-RA vsetvli insertion (#110796)

Now that LLVM 19.1.0 has been out for a while with post-vector-RA
vsetvli insertion enabled by default, this proposes to remove the flag
that restores the old pre-RA behaviour so we only have one configuration
going forward.

That flag was mainly meant as a fallback in case users ran into issues,
but I haven't seen anything reported so far.

Author: lukel97

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -179,17 +179,10 @@ bool RISCVInstrInfo::isReallyTriviallyReMaterializable(
   case RISCV::VMV_S_X:
   case RISCV::VFMV_S_F:
   case RISCV::VID_V:
-    if (MI.getOperand(1).isUndef() &&
-        /* After RISCVInsertVSETVLI most pseudos will have implicit uses on vl
-           and vtype.  Make sure we only rematerialize before RISCVInsertVSETVLI
-           i.e. -riscv-vsetvl-after-rvv-regalloc=true */
-        !MI.hasRegisterImplicitUseOperand(RISCV::VTYPE))
-      return true;
-    break;
+    return MI.getOperand(1).isUndef();
   default:
-    break;
+    return TargetInstrInfo::isReallyTriviallyReMaterializable(MI);
   }
-  return TargetInstrInfo::isReallyTriviallyReMaterializable(MI);
 }
 
 static bool forwardCopyWillClobberTuple(unsigned DstReg, unsigned SrcReg,

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

@@ -99,11 +99,6 @@ static cl::opt<bool> EnableMISchedLoadStoreClustering(
     cl::desc("Enable load and store clustering in the machine scheduler"),
     cl::init(true));
 
-static cl::opt<bool> EnableVSETVLIAfterRVVRegAlloc(
-    "riscv-vsetvl-after-rvv-regalloc", cl::Hidden,
-    cl::desc("Insert vsetvls after vector register allocation"),
-    cl::init(true));
-
 static cl::opt<bool>
     EnableVLOptimizer("riscv-enable-vl-optimizer",
                       cl::desc("Enable the RISC-V VL Optimizer pass"),
@@ -413,8 +408,7 @@ FunctionPass *RISCVPassConfig::createRVVRegAllocPass(bool Optimized) {
 
 bool RISCVPassConfig::addRegAssignAndRewriteFast() {
   addPass(createRVVRegAllocPass(false));
-  if (EnableVSETVLIAfterRVVRegAlloc)
-    addPass(createRISCVInsertVSETVLIPass());
+  addPass(createRISCVInsertVSETVLIPass());
   if (TM->getOptLevel() != CodeGenOptLevel::None &&
       EnableRISCVDeadRegisterElimination)
     addPass(createRISCVDeadRegisterDefinitionsPass());
@@ -424,8 +418,7 @@ bool RISCVPassConfig::addRegAssignAndRewriteFast() {
 bool RISCVPassConfig::addRegAssignAndRewriteOptimized() {
   addPass(createRVVRegAllocPass(true));
   addPass(createVirtRegRewriter(false));
-  if (EnableVSETVLIAfterRVVRegAlloc)
-    addPass(createRISCVInsertVSETVLIPass());
+  addPass(createRISCVInsertVSETVLIPass());
   if (TM->getOptLevel() != CodeGenOptLevel::None &&
       EnableRISCVDeadRegisterElimination)
     addPass(createRISCVDeadRegisterDefinitionsPass());
@@ -575,15 +568,6 @@ void RISCVPassConfig::addPreRegAlloc() {
   addPass(createRISCVInsertReadWriteCSRPass());
   addPass(createRISCVInsertWriteVXRMPass());
   addPass(createRISCVLandingPadSetupPass());
-
-  // Run RISCVInsertVSETVLI after PHI elimination. On O1 and above do it after
-  // register coalescing so needVSETVLIPHI doesn't need to look through COPYs.
-  if (!EnableVSETVLIAfterRVVRegAlloc) {
-    if (TM->getOptLevel() == CodeGenOptLevel::None)
-      insertPass(&PHIEliminationID, &RISCVInsertVSETVLIID);
-    else
-      insertPass(&RegisterCoalescerID, &RISCVInsertVSETVLIID);
-  }
 }
 
 void RISCVPassConfig::addFastRegAlloc() {