Eliminate duplicate shadow stack pointer loads.

Shadow prologue before:
```
define dso_local ptr @getobjname(ptr noundef %0, i32 noundef %1, i32 noundef %2, ptr noundef %3) unnamed_addr rust-lang#29 !dbg !16937 {
  call void @__yk_trace_basicblock(i32 137, i32 0)
  %5 = load ptr, ptr @shadowstack_0, align 8
  call void @llvm.dbg.value(metadata ptr %5, metadata !16961, metadata !DIExpression()), !dbg !16962
  %6 = load ptr, ptr @shadowstack_0, align 8
  %7 = getelementptr i8, ptr %6, i32 16
  store ptr %7, ptr @shadowstack_0, align 8
  ...
```

After:
```
define dso_local ptr @getobjname(ptr noundef %0, i32 noundef %1, i32 noundef %2, ptr noundef %3) unnamed_addr rust-lang#29 !dbg !16937 {
  call void @__yk_trace_basicblock(i32 137, i32 0)
  %5 = load ptr, ptr @shadowstack_0, align 8
  call void @llvm.dbg.value(metadata ptr %5, metadata !16961, metadata !DIExpression()), !dbg !16962
  %6 = getelementptr i8, ptr %5, i32 16
  store ptr %6, ptr @shadowstack_0, align 8
  ...
```

Author: vext01

llvm/lib/Transforms/Yk/ShadowStack.cpp

@@ -159,7 +159,8 @@ class YkShadowStackImpl {
   // Insert a shadowstack_ptr debug variable when compiled with '-g' so that in
   // gdb we can know the offset into the shadow stack for each frame. This uses
   // llvm's debug intrinsic so does not affect optimized code.
-  void insertShadowDbgInfo(Function &F, DataLayout &DL, size_t SFrameSize) {
+  void insertShadowDbgInfo(Function &F, DataLayout &DL, size_t SFrameSize,
+                           Value *SSPtr) {
     DISubprogram *SP = F.getSubprogram();
     if (!SP) {
       return;
@@ -176,17 +177,10 @@ class YkShadowStackImpl {
         false, // AlwaysPreserve
         DINode::DIFlags::FlagArtificial);
 
-    // And insert it at the beginning of the function
+    // And insert it after the load of the shadow stack pointer.
     DILocation *DIL = DILocation::get(F.getContext(), SP->getLine(), 0, SP);
-    Instruction *InsertBefore = &*F.getEntryBlock().getFirstInsertionPt();
-    Instruction *First = F.getEntryBlock().getFirstNonPHI();
-    IRBuilder<> Builder(First);
-
-    // Load the shadow stack pointer out of the global variable and assign it
-    // to our debug local.
-    Value *SSPtr = Builder.CreateLoad(Int8PtrTy, ShadowCurrent);
     DIB->insertDbgValueIntrinsic(SSPtr, DebugVar, DIB->createExpression(), DIL,
-                                 InsertBefore);
+                                 cast<LoadInst>(SSPtr)->getNextNode());
   }
 
   // Insert main's prologue.
@@ -474,7 +468,7 @@ class YkShadowStackImpl {
       size_t SFrameSize = analyseFunction(F, DL, Allocas, Rets);
       if (SFrameSize > 0) {
         Value *InitSSPtr = insertShadowPrologue(F, SFrameSize);
-        insertShadowDbgInfo(F, DL, SFrameSize);
+        insertShadowDbgInfo(F, DL, SFrameSize, InitSSPtr);
         rewriteAllocas(DL, Allocas, InitSSPtr);
         insertShadowEpilogues(Rets, InitSSPtr);
       }