[RISCV][MachineVerifier] Use RegUnit for register liveness checking (#115980)

For the RISC-V target, V14_V15 are not subregisters of v14m4, even
though they share some registers. Currently, the MachineVerifier reports
an error when checking register liveness for segment load/store
operations.

This patch adds additional register liveness checking, using RegUnit
instead of subregisters, to prevent this error.

Author: BeMg

llvm/lib/CodeGen/MachineVerifier.cpp

@@ -3033,7 +3033,11 @@ void MachineVerifier::checkLiveness(const MachineOperand *MO, unsigned MONum) {
             if (!MOP.getReg().isPhysical())
               continue;
 
-            if (llvm::is_contained(TRI->subregs(MOP.getReg()), Reg))
+            if (MOP.getReg() != Reg &&
+                all_of(TRI->regunits(Reg), [&](const MCRegUnit RegUnit) {
+                  return llvm::is_contained(TRI->regunits(MOP.getReg()),
+                                            RegUnit);
+                }))
               Bad = false;
           }
         }

llvm/test/MachineVerifier/RISCV/subreg-liveness.mir

@@ -0,0 +1,26 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=riscv64 -mattr=+v -run-pass=none %s -o - | FileCheck %s
+
+# During the MachineVerifier, it assumes that used registers have been defined
+# In this test case, while $v12_v13_v14_v15_v16 covers $v14_v15,
+# $v14_v15 is not a sub-register of $v14m2 even though they share the same register.
+# This corner case can be resolved by checking the register using RegUnit.
+
+...
+---
+name:            func
+tracksRegLiveness: true
+tracksDebugUserValues: true
+body:             |
+  bb.0:
+    liveins: $v0, $v8, $v9, $v10, $v11
+
+    ; CHECK-LABEL: name: func
+    ; CHECK: liveins: $v0, $v8, $v9, $v10, $v11
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: renamable $v16m2 = PseudoVMV_V_I_M2 undef renamable $v16m2, 0, -1, 3 /* e8 */, 0 /* tu, mu */, implicit $vl, implicit $vtype
+    ; CHECK-NEXT: $v20m2 = VMV2R_V $v14m2, implicit $v12_v13_v14_v15_v16
+    renamable $v16m2 = PseudoVMV_V_I_M2 undef renamable $v16m2, 0, -1, 3 /* e8 */, 0 /* tu, mu */, implicit $vl, implicit $vtype
+    $v20m2 = VMV2R_V $v14m2, implicit $v12_v13_v14_v15_v16
+
+...