[InstCombine] Compute result directly on APInts

If the bitwidth is 2 and we add two 1s, the result may overflow.
This is fine in terms of correctness, but triggers the APInt ctor
assertion. Fix this by performing the calculation directly on APInts.

Fixes the issue reported in:
llvm/llvm-project#114539 (comment)

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -3038,12 +3038,12 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
     unsigned BW = C.getBitWidth();
     std::bitset<4> Table;
     auto ComputeTable = [&](bool Op0Val, bool Op1Val) {
-      int Res = 0;
+      APInt Res(BW, 0);
       if (Op0Val)
-        Res += isa<ZExtInst>(Ext0) ? 1 : -1;
+        Res += APInt(BW, isa<ZExtInst>(Ext0) ? 1 : -1, /*isSigned=*/true);
       if (Op1Val)
-        Res += isa<ZExtInst>(Ext1) ? 1 : -1;
-      return ICmpInst::compare(APInt(BW, Res, true), C, Pred);
+        Res += APInt(BW, isa<ZExtInst>(Ext1) ? 1 : -1, /*isSigned=*/true);
+      return ICmpInst::compare(Res, C, Pred);
     };
 
     Table[0] = ComputeTable(false, false);

llvm/test/Transforms/InstCombine/icmp-add.ll

@@ -79,6 +79,19 @@ bb:
   ret i1 %i4
 }
 
+define i1 @cvt_icmp_0_zext_plus_zext_eq_i2(i1 %a, i1 %b) {
+; CHECK-LABEL: @cvt_icmp_0_zext_plus_zext_eq_i2(
+; CHECK-NEXT:    [[TMP1:%.*]] = or i1 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = xor i1 [[TMP1]], true
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %a.ext = zext i1 %a to i2
+  %b.ext = zext i1 %b to i2
+  %add = add i2 %a.ext, %b.ext
+  %cmp = icmp eq i2 %add, 0
+  ret i1 %cmp
+}
+
 define i1 @cvt_icmp_1_zext_plus_zext_eq(i1 %arg, i1 %arg1) {
 ; CHECK-LABEL: @cvt_icmp_1_zext_plus_zext_eq(
 ; CHECK-NEXT:  bb: