[ARM] Fix APInt assert for CSNEG known bits.

Use APInt::getAllOnes(32), to avoid the assert when constructing an APInt
from -1.

Author: davemgreen

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -20138,8 +20138,8 @@ void ARMTargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
     else if (Op.getOpcode() == ARMISD::CSINV)
       std::swap(KnownOp1.Zero, KnownOp1.One);
     else if (Op.getOpcode() == ARMISD::CSNEG)
-      KnownOp1 = KnownBits::mul(
-          KnownOp1, KnownBits::makeConstant(APInt(32, -1)));
+      KnownOp1 = KnownBits::mul(KnownOp1,
+                                KnownBits::makeConstant(APInt::getAllOnes(32)));
 
     Known = KnownOp0.intersectWith(KnownOp1);
     break;

llvm/test/CodeGen/Thumb2/csel.ll

@@ -364,3 +364,18 @@ entry:
   %t = xor i32 %s, %spec.select
   ret i32 %t
 }
+
+define i32 @test_known_bits(i32 %a, i32 %b) {
+; CHECK-LABEL: test_known_bits:
+; CHECK:       @ %bb.0: @ %entry
+; CHECK-NEXT:    movs r2, #1
+; CHECK-NEXT:    cmp r0, #0
+; CHECK-NEXT:    cneg r0, r2, mi
+; CHECK-NEXT:    add r0, r1
+; CHECK-NEXT:    bx lr
+entry:
+  %c = icmp slt i32 %a, 0
+  %s = select i1 %c, i32 -1, i32 1
+  %t = add i32 %s, %b
+  ret i32 %t
+}