[RISCV][VLOPT][NFC] Remove outdated FIXME comments related to supported instructions (#156126)

Remove several FIXME comments in `isSupportedInstr` for opcodes that
were already implemented. Also moved switch cases for
add-carry/sub-borrow instructions together.

NFC.

Author: mshockwave

llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

@@ -900,13 +900,6 @@ static bool isSupportedInstr(const MachineInstr &MI) {
   case RISCV::VSEXT_VF4:
   case RISCV::VZEXT_VF8:
   case RISCV::VSEXT_VF8:
-  // Vector Integer Add-with-Carry / Subtract-with-Borrow Instructions
-  // FIXME: Add support
-  case RISCV::VMADC_VV:
-  case RISCV::VMADC_VI:
-  case RISCV::VMADC_VX:
-  case RISCV::VMSBC_VV:
-  case RISCV::VMSBC_VX:
   // Vector Narrowing Integer Right Shift Instructions
   case RISCV::VNSRL_WX:
   case RISCV::VNSRL_WI:
@@ -993,6 +986,11 @@ static bool isSupportedInstr(const MachineInstr &MI) {
   case RISCV::VSBC_VXM:
   case RISCV::VMSBC_VVM:
   case RISCV::VMSBC_VXM:
+  case RISCV::VMADC_VV:
+  case RISCV::VMADC_VI:
+  case RISCV::VMADC_VX:
+  case RISCV::VMSBC_VV:
+  case RISCV::VMSBC_VX:
   // Vector Widening Integer Multiply-Add Instructions
   case RISCV::VWMACCU_VV:
   case RISCV::VWMACCU_VX:
@@ -1001,10 +999,7 @@ static bool isSupportedInstr(const MachineInstr &MI) {
   case RISCV::VWMACCSU_VV:
   case RISCV::VWMACCSU_VX:
   case RISCV::VWMACCUS_VX:
-  // Vector Integer Merge Instructions
-  // FIXME: Add support
   // Vector Integer Move Instructions
-  // FIXME: Add support
   case RISCV::VMV_V_I:
   case RISCV::VMV_V_X:
   case RISCV::VMV_V_V: