Use accessors for ValueHandleBase::V; NFC

[lizhengxing]

This PR pulls the upstream change, Use accessors for ValueHandleBase::V; NFC (llvm/llvm-project@6f08789), into DXC.

Here's the summary of the change:

This changes code that touches ValueHandleBase::V to go through getValPtr and (newly added) setValPtr. This functionality will be used later, but also seemed like a generally good cleanup.

I also renamed the field to Val, but that's just to make it obvious that I fixed all the uses.

This is part 1 of the fix for #6659.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 978d36221daecfb745fde05a78bd134a389cb934 2378f9b705bdabc953a707b4abcef16d9c177c4f -- include/llvm/IR/ValueHandle.h lib/IR/Value.cpp

View the diff from clang-format here.

diff --git a/include/llvm/IR/ValueHandle.h b/include/llvm/IR/ValueHandle.h
index 6a9a9cf4..08a59806 100644
--- a/include/llvm/IR/ValueHandle.h
+++ b/include/llvm/IR/ValueHandle.h
@@ -63,9 +63,9 @@ private:
   ValueHandleBase(const ValueHandleBase&) = delete;
 public:
   explicit ValueHandleBase(HandleBaseKind Kind)
-    : PrevPair(nullptr, Kind), Next(nullptr), Val(nullptr) {}
+      : PrevPair(nullptr, Kind), Next(nullptr), Val(nullptr) {}
   ValueHandleBase(HandleBaseKind Kind, Value *V)
-    : PrevPair(nullptr, Kind), Next(nullptr), Val(V) {
+      : PrevPair(nullptr, Kind), Next(nullptr), Val(V) {
     if (isValid(getValPtr()))
       AddToUseList();
   }

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