[RISCV] Remove -riscv-disable-insert-vsetvl-phi-opt flag (#97991)

This flag was added in https://reviews.llvm.org/D103277 out of
precaution, but it's been enabled by default for 3 years now and I
haven't seen any miscompiles upstream stemming from needVSETVLIPHI. This
would remove it just to simplify the number of configurations.

Author: lukel97

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -41,10 +41,6 @@ using namespace llvm;
 STATISTIC(NumInsertedVSETVL, "Number of VSETVL inst inserted");
 STATISTIC(NumCoalescedVSETVL, "Number of VSETVL inst coalesced");
 
-static cl::opt<bool> DisableInsertVSETVLPHIOpt(
-    "riscv-disable-insert-vsetvl-phi-opt", cl::init(false), cl::Hidden,
-    cl::desc("Disable looking through phis when inserting vsetvlis."));
-
 namespace {
 
 /// Given a virtual register \p Reg, return the corresponding VNInfo for it.
@@ -1364,9 +1360,6 @@ void RISCVInsertVSETVLI::computeIncomingVLVTYPE(const MachineBasicBlock &MBB) {
 // outputs from the last VSETVLI in their respective basic blocks.
 bool RISCVInsertVSETVLI::needVSETVLIPHI(const VSETVLIInfo &Require,
                                         const MachineBasicBlock &MBB) const {
-  if (DisableInsertVSETVLPHIOpt)
-    return true;
-
   if (!Require.hasAVLReg())
     return true;