[InferAddressSpaces] Mark ConstantAggregateZero as safe to cast to a ConstantExpr addrspacecast (llvm#159695)

This PR extends isSafeToCastConstAddrSpace to treat
ConstantAggregateZero like ConstantPointerNull.
Tests shows an extra addrspacecast instruction is removed and icmp
pointer vector operand's address space is now inferred.

This change is motivated by inspecting the test in commit f7629f5.

Author: wenju-he

llvm/lib/Transforms/Scalar/InferAddressSpaces.cpp

@@ -1170,7 +1170,7 @@ bool InferAddressSpacesImpl::isSafeToCastConstAddrSpace(Constant *C,
   if (SrcAS != FlatAddrSpace && NewAS != FlatAddrSpace)
     return false;
 
-  if (isa<ConstantPointerNull>(C))
+  if (isa<ConstantPointerNull>(C) || isa<ConstantAggregateZero>(C))
     return true;
 
   if (auto *Op = dyn_cast<Operator>(C)) {

llvm/test/Transforms/InferAddressSpaces/AMDGPU/issue110433.ll

@@ -6,8 +6,7 @@ define <8 x i1> @load_vector_of_flat_ptr_from_constant(ptr addrspace(4) %ptr) {
 ; CHECK-SAME: ptr addrspace(4) [[PTR:%.*]]) {
 ; CHECK-NEXT:    [[LD:%.*]] = load <8 x ptr>, ptr addrspace(4) [[PTR]], align 128
 ; CHECK-NEXT:    [[TMP1:%.*]] = addrspacecast <8 x ptr> [[LD]] to <8 x ptr addrspace(1)>
-; CHECK-NEXT:    [[TMP2:%.*]] = addrspacecast <8 x ptr addrspace(1)> [[TMP1]] to <8 x ptr>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <8 x ptr> [[TMP2]], zeroinitializer
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <8 x ptr addrspace(1)> [[TMP1]], <ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1)), ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1)), ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1)), ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1)), ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1)), ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1)), ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1)), ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1))>
 ; CHECK-NEXT:    ret <8 x i1> [[CMP]]
 ;
   %ld = load <8 x ptr>, ptr addrspace(4) %ptr, align 128

llvm/test/Transforms/InferAddressSpaces/AMDGPU/phi-poison.ll

@@ -11,9 +11,8 @@ define void @phi_poison(ptr addrspace(1) %arg, <2 x ptr addrspace(1)> %arg1) {
 ; CHECK:       merge:
 ; CHECK-NEXT:    [[I:%.*]] = phi ptr addrspace(1) [ [[ARG:%.*]], [[LEADER]] ], [ poison, [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    [[I2:%.*]] = phi <2 x ptr addrspace(1)> [ [[ARG1:%.*]], [[LEADER]] ], [ poison, [[ENTRY]] ]
-; CHECK-NEXT:    [[TMP1:%.*]] = addrspacecast <2 x ptr addrspace(1)> [[I2]] to <2 x ptr>
 ; CHECK-NEXT:    [[J:%.*]] = icmp eq ptr addrspace(1) [[I]], addrspacecast (ptr null to ptr addrspace(1))
-; CHECK-NEXT:    [[J1:%.*]] = icmp eq <2 x ptr> [[TMP1]], zeroinitializer
+; CHECK-NEXT:    [[J1:%.*]] = icmp eq <2 x ptr addrspace(1)> [[I2]], <ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1)), ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1))>
 ; CHECK-NEXT:    ret void
 ;
 entry: