[llvm] select v_sat_pk from two i16 or v2i16 (PR #121124)

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Shoreshen wrote:

> Can you please add tests for GFX12 and implementation for GFX11 and GFX12? The V_SAT_PK_U8_I16 instruction exists on those subtargets as V_SAT_PK_U8_I16_fake16 and V_SAT_PK_U8_I16_t16. V_SAT_PK_U8_I16_fake16 should work equivalently to gfx9 and should work now. A true16 version using V_SAT_PK_U8_I16_t16 may or may not be testable at the current time, and could make sense to defer.

Hi, it seems like the t16 instruction has more than 1 operand, so the patterns doesn't fit..... I added for the fake16 instructions, and also checks for GFX12

https://github.com/llvm/llvm-project/pull/121124