[RISCV][NFC] Fix UBSan issue in ISel (#181422)

If `int Imm` is `INT64_MIN`, then negating this value overflows, which
is undefined behaviour. This adds this case to the tests, and avoids
this case.

Co-authored-by: Sampath Vutkoori <svutkoor@qti.qualcomm.com>

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -4067,6 +4067,8 @@ bool RISCVDAGToDAGISel::selectNegImm(SDValue N, SDValue &Val) {
   int64_t Imm = cast<ConstantSDNode>(N)->getSExtValue();
   if (isInt<32>(Imm))
     return false;
+  if (Imm == INT64_MIN)
+    return false;
 
   for (const SDNode *U : N->users()) {
     switch (U->getOpcode()) {

llvm/test/CodeGen/RISCV/add-imm64-to-sub.ll

@@ -74,3 +74,21 @@ define i64 @add_multiuse_const(i64 %x, i64 %y) {
   %xor = xor i64 %a, %b
   ret i64 %xor
 }
+
+
+define i64 @add_i64_min(i64 %x) {
+; NOZBS-LABEL: add_i64_min:
+; NOZBS:       # %bb.0:
+; NOZBS-NEXT:    li a1, -1
+; NOZBS-NEXT:    slli a1, a1, 63
+; NOZBS-NEXT:    add a0, a0, a1
+; NOZBS-NEXT:    ret
+;
+; ZBS-LABEL: add_i64_min:
+; ZBS:       # %bb.0:
+; ZBS-NEXT:    bseti a1, zero, 63
+; ZBS-NEXT:    add a0, a0, a1
+; ZBS-NEXT:    ret
+  %a = add i64 %x, -9223372036854775808
+  ret i64 %a
+}