[DirectX backend] avoid generate redundant bitcast in DXILPrepareModu…

…le (#65163)

When emit NoOp bitcast for GEP Ptr Operand, should use SourceElementType
instead of ResultElementType.

**Behavior Before Change**
Redundant bitcast like 
   ` bitcast ptr addrspace(3) @gs to ptr addrspace(3)`
 will be generated for llvm/test/CodeGen/DirectX/typed_ptr.ll

**Behavior After Change**
  No bitcast will be generated.

Fixes #65183

llvm/lib/Target/DirectX/DXILPrepare.cpp

@@ -154,7 +154,7 @@ class DXILPrepareModule : public ModulePass {
           if (auto GEP = dyn_cast<GetElementPtrInst>(&I)) {
             if (Value *NoOpBitcast = maybeGenerateBitcast(
                     Builder, PointerTypes, I, GEP->getPointerOperand(),
-                    GEP->getResultElementType()))
+                    GEP->getSourceElementType()))
               GEP->setOperand(0, NoOpBitcast);
             continue;
           }

llvm/test/CodeGen/DirectX/typed_ptr.ll

@@ -1,10 +1,29 @@
 ; RUN: opt -S -dxil-prepare < %s | FileCheck %s
 target triple = "dxil-unknown-unknown"
 
-; Make sure not crash when has typed ptr.
-; CHECK:@test
+@gs = external addrspace(3) global [20 x [6 x float]], align 4
 
+; Make sure not crash when has typed ptr.
 define i64 @test(i64* %p) {
+; CHECK-LABEL: define i64 @test(
+; CHECK-SAME: ptr [[P:%.*]]) {
+; CHECK-NEXT:    [[V:%.*]] = load i64, ptr [[P]], align 4
+; CHECK-NEXT:    ret i64 [[V]]
+;
   %v = load i64, i64* %p
   ret i64 %v
 }
+
+; Make sure no bitcast generated.
+define void @test_gep() {
+; CHECK-LABEL: define void @test_gep() {
+; CHECK-NEXT:    [[BASE:%.*]] = getelementptr inbounds [20 x [6 x float]], ptr addrspace(3) @gs, i64 0, i64 3
+; CHECK-NEXT:    [[ADDR:%.*]] = getelementptr inbounds [6 x float], ptr addrspace(3) [[BASE]], i64 0, i64 2
+; CHECK-NEXT:    store float 1.000000e+00, ptr addrspace(3) [[ADDR]], align 4
+; CHECK-NEXT:    ret void
+;
+  %base = getelementptr inbounds [20 x [6 x float]], ptr addrspace(3) @gs, i64 0, i64 3
+  %addr = getelementptr inbounds [6 x float], ptr addrspace(3) %base, i64 0, i64 2
+  store float 1.000000e+00, ptr addrspace(3) %addr, align 4
+  ret void
+}