[clang] First draft: [Clang] Add constant evaluation support for x86 psadbw (PR #169253)

[Simon Pilgrim via cfe-commits]

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@@ -173,13 +173,13 @@ let Features = "sse2", Attributes = [NoThrow] in {
 let Features = "sse2", Attributes = [NoThrow, Const, Constexpr, RequiredVectorWidth<128>] in {
   def cvtpd2ps : X86Builtin<"_Vector<4, float>(_Vector<2, double>)">;
   def cvtsd2ss : X86Builtin<"_Vector<4, float>(_Vector<4, float>, _Vector<2, double>)">;
+  def psadbw128 : X86Builtin<"_Vector<2, long long int>(_Vector<16, char>, _Vector<16, char>)">;
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RKSimon wrote:

we should probably update the signature to:
```suggestion
  def psadbw128 : X86Builtin<"_Vector<2, long long int>(_Vector<16, unsigned char>, _Vector<16, unsigned char>)">;
```
same for the others (might require changes to the casts in the headers)

https://github.com/llvm/llvm-project/pull/169253