[AMDGPU][Legalizer] Avoid pack/unpack for G_FSHR (#156796)

Scalarize G_FSHR only if the subtarget does not support V2S16 type.

Author: gandhi56

llvm/lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp

@@ -2082,13 +2082,12 @@ AMDGPULegalizerInfo::AMDGPULegalizerInfo(const GCNSubtarget &ST_,
     .scalarize(0)
     .lower();
 
-  // TODO: Only Try to form v2s16 with legal packed instructions.
-  getActionDefinitionsBuilder(G_FSHR)
-    .legalFor({{S32, S32}})
-    .lowerFor({{V2S16, V2S16}})
-    .clampMaxNumElementsStrict(0, S16, 2)
-    .scalarize(0)
-    .lower();
+  auto &FSHRActionDefs = getActionDefinitionsBuilder(G_FSHR);
+  FSHRActionDefs.legalFor({{S32, S32}})
+                              .clampMaxNumElementsStrict(0, S16, 2);
+  if (ST.hasVOP3PInsts())
+    FSHRActionDefs.lowerFor({{V2S16, V2S16}});
+  FSHRActionDefs.scalarize(0).lower();
 
   if (ST.hasVOP3PInsts()) {
     getActionDefinitionsBuilder(G_FSHL)