[InstCombine] Fold gep of exact unsigned division (#82334)

Extend the transform added in
#76458 to also handle unsigned
division. X exact/ Y * Y == X holds independently of whether the
division is signed or unsigned.

Proofs: https://alive2.llvm.org/ce/z/wFd5Ec
(cherry picked from commit 26d4afc)

Author: nikic

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2594,10 +2594,10 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
         Value *V;
         if ((has_single_bit(TyAllocSize) &&
              match(GEP.getOperand(1),
-                   m_Exact(m_AShr(m_Value(V),
-                                  m_SpecificInt(countr_zero(TyAllocSize)))))) ||
+                   m_Exact(m_Shr(m_Value(V),
+                                 m_SpecificInt(countr_zero(TyAllocSize)))))) ||
             match(GEP.getOperand(1),
-                  m_Exact(m_SDiv(m_Value(V), m_SpecificInt(TyAllocSize))))) {
+                  m_Exact(m_IDiv(m_Value(V), m_SpecificInt(TyAllocSize))))) {
           GetElementPtrInst *NewGEP = GetElementPtrInst::Create(
               Builder.getInt8Ty(), GEP.getPointerOperand(), V);
           NewGEP->setIsInBounds(GEP.isInBounds());

llvm/test/Transforms/InstCombine/getelementptr.ll

@@ -1476,8 +1476,7 @@ define ptr @gep_sdiv(ptr %p, i64 %off) {
 
 define ptr @gep_udiv(ptr %p, i64 %off) {
 ; CHECK-LABEL: @gep_udiv(
-; CHECK-NEXT:    [[INDEX:%.*]] = udiv exact i64 [[OFF:%.*]], 7
-; CHECK-NEXT:    [[PTR:%.*]] = getelementptr [[STRUCT_C:%.*]], ptr [[P:%.*]], i64 [[INDEX]]
+; CHECK-NEXT:    [[PTR:%.*]] = getelementptr i8, ptr [[P:%.*]], i64 [[OFF:%.*]]
 ; CHECK-NEXT:    ret ptr [[PTR]]
 ;
   %index = udiv exact i64 %off, 7
@@ -1517,8 +1516,7 @@ define ptr @gep_ashr(ptr %p, i64 %off) {
 
 define ptr @gep_lshr(ptr %p, i64 %off) {
 ; CHECK-LABEL: @gep_lshr(
-; CHECK-NEXT:    [[INDEX:%.*]] = lshr exact i64 [[OFF:%.*]], 2
-; CHECK-NEXT:    [[PTR:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[INDEX]]
+; CHECK-NEXT:    [[PTR:%.*]] = getelementptr i8, ptr [[P:%.*]], i64 [[OFF:%.*]]
 ; CHECK-NEXT:    ret ptr [[PTR]]
 ;
   %index = lshr exact i64 %off, 2