Fix handling of complex constant expressions

When some constant expression is used as operand of regular instruction
and operand of other constant expression, the current algorithm of
lowering could produce incorrect order of instructions because it is not
possible to add instruction operand to a constant expression. Consider
following pseudo code example:
```
call foo(constexpr2(constexpr1), constexpr1)

// After first loop iteration through operands of the call instruction:

A = constexpr2op(constexpr1)
call foo(A, constexpr1)

// Ok, instruction A is now a user of constexpr1, so, when second
// operand is processed, all uses of constexpr1 are updated, but the
// instruction that represents constexpr1 is inserted before call
// instruction because it is now being processed, so it will look like
// this:

A = constexpr2(B)
B = constexpr1
call foo(A, B)

// So, instruction B needs to be moved after all its users to get a
// valid module:

B = constexpr1
A = constexpr2(B)
call foo(A, B)

```

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@390aba9

Author: Fznamznon

llvm-spirv/lib/SPIRV/SPIRVLowerConstExpr.cpp

@@ -157,8 +157,12 @@ void SPIRVLowerConstExprBase::visit(Module *M) {
               Users.push_back(InstUser);
           }
         }
-        for (auto &User : Users)
+        for (auto &User : Users) {
+          if (ReplInst->getParent() == User->getParent())
+            if (User->comesBefore(ReplInst))
+              ReplInst->moveBefore(User);
           User->replaceUsesOfWith(CE, ReplInst);
+        }
         return ReplInst;
       };
 

llvm-spirv/test/complex-constexpr.ll

@@ -0,0 +1,43 @@
+; RUN: llvm-as %s -o %t.bc
+; RUN: llvm-spirv %t.bc -o %t.spv
+; RUN: llvm-spirv %t.spv -o %t.spt --to-text
+; RUN: llvm-spirv -r %t.spv -o %t.bc
+; RUN: llvm-dis %t.bc -o %t.ll
+; RUN: FileCheck %s --input-file %t.spt -check-prefix=CHECK-SPIRV
+; RUN: FileCheck %s --input-file %t.ll  -check-prefix=CHECK-LLVM
+; RUN: spirv-val %t.spv
+
+target datalayout = "e-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-n8:16:32:64"
+target triple = "spir64"
+
+@.str.1 = private unnamed_addr addrspace(1) constant [1 x i8] zeroinitializer, align 1
+
+define linkonce_odr hidden spir_func void @foo() {
+entry:
+; CHECK-SPIRV: PtrCastToGeneric {{[0-9]+}} [[Cast:[0-9]+]]
+; CHECK-SPIRV: InBoundsPtrAccessChain {{[0-9]+}} [[Gep:[0-9]+]] [[Cast]]
+; CHECK-SPIRV: InBoundsPtrAccessChain {{[0-9]+}} [[Gep1:[0-9]+]] [[Cast]]
+; CHECK-SPIRV: ConvertPtrToU {{[0-9]+}} [[PtrToU1:[0-9]+]] [[Gep1]]
+; CHECK-SPIRV: ConvertPtrToU {{[0-9]+}} [[PtrToU2:[0-9]+]]
+; CHECK-SPIRV: IEqual {{[0-9]+}} [[IEq:[0-9]+]] [[PtrToU1]] [[PtrToU2]]
+; CHECK-SPIRV: ConvertUToPtr {{[0-9]+}} [[UToPtr:[0-9]+]]
+; CHECK-SPIRV: Select {{[0-9]+}} [[Sel:[0-9]+]] [[IEq]] [[UToPtr]] [[Gep1]]
+; CHECK-SPIRV: FunctionCall 9 31 28 [[Gep]] [[Sel]]
+; CHECK-LLVM:  %[[Cast:[0-9]+]] = addrspacecast [1 x i8] addrspace(1)* @.str.1 to [1 x i8] addrspace(4)*
+; CHECK-LLVM:  %[[Gep:[0-9]+]] = getelementptr inbounds [1 x i8], [1 x i8] addrspace(4)* %0, i64 0, i64 0
+; CHECK-LLVM:  %[[Gep1:[0-9]+]] = getelementptr inbounds [1 x i8], [1 x i8] addrspace(4)* %0, i64 0, i64 0
+; CHECK-LLVM:  %[[PtrToU1:[0-9]+]] = ptrtoint i8 addrspace(4)* %[[Gep1]] to i64
+; CHECK-LLVM:  %[[PtrToU2:[0-9]+]] = ptrtoint i8 addrspace(4)* null to i64
+; CHECK-LLVM:  %[[IEq:[0-9]+]] = icmp eq i64 %[[PtrToU1]], %[[PtrToU2]]
+; CHECK-LLVM:  %[[UToPtr:[0-9]+]] = inttoptr i64 -1 to i8 addrspace(4)*
+; CHECK-LLVM:  %[[Sel:[0-9]+]] = select i1 %[[IEq]], i8 addrspace(4)* %[[UToPtr]], i8 addrspace(4)* %[[Gep1]]
+; CHECK-LLVM:  call spir_func void @bar(i8 addrspace(4)* %[[Gep]], i8 addrspace(4)* %[[Sel]]) #0
+  call spir_func void @bar(i8 addrspace(4)* getelementptr inbounds ([1 x i8], [1 x i8] addrspace(4)* addrspacecast ([1 x i8] addrspace(1)* @.str.1 to [1 x i8] addrspace(4)*), i64 0, i64 0), i8 addrspace(4)* select (i1 icmp eq (i8 addrspace(4)* getelementptr inbounds ([1 x i8], [1 x i8] addrspace(4)* addrspacecast ([1 x i8] addrspace(1)* @.str.1 to [1 x i8] addrspace(4)*), i64 0, i64 0), i8 addrspace(4)* null), i8 addrspace(4)* inttoptr (i64 -1 to i8 addrspace(4)*), i8 addrspace(4)* getelementptr inbounds ([1 x i8], [1 x i8] addrspace(4)* addrspacecast ([1 x i8] addrspace(1)* @.str.1 to [1 x i8] addrspace(4)*), i64 0, i64 0)))
+  ret void
+}
+
+define linkonce_odr hidden spir_func void @bar(i8 addrspace(4)* %__beg, i8 addrspace(4)* %__end) {
+entry:
+  ret void
+}
+