[InstCombine] Intersect nowrap flags between geps while folding into phi

A miscompilation issue has been addressed with refined checking.

Fixes: #115149.

Author: antoniofrighetto

llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp

@@ -545,7 +545,8 @@ Instruction *InstCombinerImpl::foldPHIArgGEPIntoPHI(PHINode &PN) {
   // especially bad when the PHIs are in the header of a loop.
   bool NeededPhi = false;
 
-  GEPNoWrapFlags NW = GEPNoWrapFlags::all();
+  // Remember flags of the first phi-operand getelementptr.
+  GEPNoWrapFlags NW = FirstInst->getNoWrapFlags();
 
   // Scan to see if all operands are the same opcode, and all have one user.
   for (Value *V : drop_begin(PN.incoming_values())) {

llvm/test/Transforms/InstCombine/opaque-ptr.ll

@@ -549,7 +549,7 @@ define ptr @phi_of_gep_flags_1(i1 %c, ptr %p) {
 ; CHECK:       else:
 ; CHECK-NEXT:    br label [[JOIN]]
 ; CHECK:       join:
-; CHECK-NEXT:    [[PHI:%.*]] = getelementptr nusw nuw i8, ptr [[P:%.*]], i64 4
+; CHECK-NEXT:    [[PHI:%.*]] = getelementptr nusw i8, ptr [[P:%.*]], i64 4
 ; CHECK-NEXT:    ret ptr [[PHI]]
 ;
   br i1 %c, label %if, label %else

llvm/test/Transforms/InstCombine/phi.ll

@@ -2795,14 +2795,13 @@ BB5:                                             ; preds = %BB4
   ret void
 }
 
-; FIXME: This is a miscompilation.
 define i64 @wrong_gep_arg_into_phi(ptr noundef %ptr) {
 ; CHECK-LABEL: @wrong_gep_arg_into_phi(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[FOR_COND:%.*]]
 ; CHECK:       for.cond:
 ; CHECK-NEXT:    [[PTR_PN:%.*]] = phi ptr [ [[PTR:%.*]], [[ENTRY:%.*]] ], [ [[DOTPN:%.*]], [[FOR_COND]] ]
-; CHECK-NEXT:    [[DOTPN]] = getelementptr inbounds nuw i8, ptr [[PTR_PN]], i64 1
+; CHECK-NEXT:    [[DOTPN]] = getelementptr i8, ptr [[PTR_PN]], i64 1
 ; CHECK-NEXT:    [[VAL:%.*]] = load i8, ptr [[DOTPN]], align 1
 ; CHECK-NEXT:    [[COND_NOT:%.*]] = icmp eq i8 [[VAL]], 0
 ; CHECK-NEXT:    br i1 [[COND_NOT]], label [[EXIT:%.*]], label [[FOR_COND]]