[InstCombine] Handle ConstantFoldCompareInstOperands() failure

This function will return nullptr instead of returning a constant
expression now, so be sure to handle that.

Fixes #93017.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -214,6 +214,9 @@ Instruction *InstCombinerImpl::foldCmpLoadFromIndexedGlobal(
     // Find out if the comparison would be true or false for the i'th element.
     Constant *C = ConstantFoldCompareInstOperands(ICI.getPredicate(), Elt,
                                                   CompareRHS, DL, &TLI);
+    if (!C)
+      return nullptr;
+
     // If the result is undef for this element, ignore it.
     if (isa<UndefValue>(C)) {
       // Extend range state machines to cover this element in case there is an

llvm/test/Transforms/InstCombine/load-cmp.ll

@@ -334,3 +334,20 @@ define i1 @test10_struct_arr_noinbounds_i64(i64 %x) {
   %r = icmp eq i32 %q, 9
   ret i1 %r
 }
+
+@table = internal constant [2 x ptr] [ptr @g, ptr getelementptr (i8, ptr @g, i64 4)], align 16
+@g = external global [2 x i32]
+
+define i1 @pr93017(i64 %idx) {
+; CHECK-LABEL: @pr93017(
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i64 [[IDX:%.*]] to i32
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds [2 x ptr], ptr @table, i32 0, i32 [[TMP1]]
+; CHECK-NEXT:    [[V:%.*]] = load ptr, ptr [[GEP]], align 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne ptr [[V]], null
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %gep = getelementptr inbounds [2 x ptr], ptr @table, i64 0, i64 %idx
+  %v = load ptr, ptr %gep
+  %cmp = icmp ne ptr %v, null
+  ret i1 %cmp
+}