[llvm] select v_sat_pk from 2 i16 (PR #121124)

[Matt Arsenault via llvm-commits]

================
@@ -3298,6 +3298,18 @@ def : GCNPat <
   (v2i16 (V_LSHL_OR_B32_e64 $src1, (i32 16), (i32 (V_AND_B32_e64 (i32 (V_MOV_B32_e32 (i32 0xffff))), $src0))))
 >;
 
+multiclass V_SAT_PK_Pat<Instruction inst> {
+    def: GCNPat<
+        (i16 (conc_lo_u8_i16 (clamp_s16_u8 i16:$lo), (clamp_s16_u8 i16:$hi))),
+        (inst
+            (V_LSHL_OR_B32_e64 VGPR_32:$hi, (S_MOV_B32 (i32 16)),
+            (V_AND_B32_e64 VGPR_32:$lo, (S_MOV_B32 (i32 0xFFFF)))))
----------------
arsenm wrote:

This feels like it should have had a dag combine form a nicer pattern to start with

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