[llvm] [AMDGPU][Legalizer] Widen i16 G_SEXT_INREG (PR #131308)

[Matt Arsenault via llvm-commits]

================
@@ -484,6 +480,78 @@ body: |
 
 ...
 
+---
+name: test_sext_inreg_s8_1
+body: |
+  bb.0:
+    liveins: $vgpr0
+
+    ; GFX9-LABEL: name: test_sext_inreg_s8_1
+    ; GFX9: liveins: $vgpr0
+    ; GFX9-NEXT: {{  $}}
+    ; GFX9-NEXT: [[COPY:%[0-9]+]]:_(s32) = COPY $vgpr0
+    ; GFX9-NEXT: [[SEXT_INREG:%[0-9]+]]:_(s32) = G_SEXT_INREG [[COPY]], 1
+    ; GFX9-NEXT: [[TRUNC:%[0-9]+]]:_(s8) = G_TRUNC [[SEXT_INREG]](s32)
+    ; GFX9-NEXT: S_ENDPGM 0, implicit [[TRUNC]](s8)
+    ;
+    ; GFX8-LABEL: name: test_sext_inreg_s8_1
+    ; GFX8: liveins: $vgpr0
+    ; GFX8-NEXT: {{  $}}
+    ; GFX8-NEXT: [[COPY:%[0-9]+]]:_(s32) = COPY $vgpr0
+    ; GFX8-NEXT: [[SEXT_INREG:%[0-9]+]]:_(s32) = G_SEXT_INREG [[COPY]], 1
+    ; GFX8-NEXT: [[TRUNC:%[0-9]+]]:_(s8) = G_TRUNC [[SEXT_INREG]](s32)
+    ; GFX8-NEXT: S_ENDPGM 0, implicit [[TRUNC]](s8)
+    ;
+    ; GFX6-LABEL: name: test_sext_inreg_s8_1
+    ; GFX6: liveins: $vgpr0
+    ; GFX6-NEXT: {{  $}}
+    ; GFX6-NEXT: [[COPY:%[0-9]+]]:_(s32) = COPY $vgpr0
+    ; GFX6-NEXT: [[SEXT_INREG:%[0-9]+]]:_(s32) = G_SEXT_INREG [[COPY]], 1
+    ; GFX6-NEXT: [[TRUNC:%[0-9]+]]:_(s8) = G_TRUNC [[SEXT_INREG]](s32)
+    ; GFX6-NEXT: S_ENDPGM 0, implicit [[TRUNC]](s8)
+    %0:_(s32) = COPY $vgpr0
+    %1:_(s8) = G_TRUNC %0
+    %2:_(s8) = G_SEXT_INREG %1, 1
+    S_ENDPGM 0, implicit %2
----------------
arsenm wrote:

Should avoid using illegally typed implicit uses like this (I've wanted to make this a machine verifier error). You should insert a G_ANYEXT to s32 and implicitly use that or copy to a physical register 

https://github.com/llvm/llvm-project/pull/131308