[X86] Allow PSHUFD/PSHUFLW/PSHUFW intrinsics in constexpr. (#161210) The i16/i32 shuffle intrinsics (`pshufw`, `pshuflw`, `pshufhw`, `pshufd`) currently cannot be used in constant expressions. This patch adds support in both bytecode interpreter (InterpBuiltin.cpp) and constant evaluator (ExprConstant.cpp) for pshuf intrinsics, enabling their use in constant expressions. ## Intrinsics covered - `_mm_shuffle_pi16` (MMX `pshufw`) - `_mm_shufflelo_epi16` / `_mm_shufflehi_epi16` - `_mm_shuffle_epi32` - Their AVX2/AVX512 vector-width variants - Masked and maskz forms (handled indirectly via `__builtin_ia32_select*`) Fixes #156611
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