AMDGPU/GlobalISel: Remove unnecessary legality constraint for G_EXTRACT

[tstellar]

Summary

We were marking G_EXTRACT operations unsupported if the output type
was larger than the input type. I don't see how this could ever actually
happen, so I dropped the constraint. Doing this makes it possible to
reuse the same legality code for G_INSERT.

[arsenm]

LGTM

lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp

getActionDefinitionsBuilder(Op)
           return VecTy.getSizeInBits() % 32 == 0 &&
             VecTy.getSizeInBits() <= 512 &&
             IdxTy.getSizeInBits() == 32;
         });
   }
 
   // FIXME: Doesn't handle extract of illegal sizes.
   getActionDefinitionsBuilder(G_EXTRACT)
-    .unsupportedIf([=](const LegalityQuery &Query) {
-        return Query.Types[0].getSizeInBits() >= Query.Types[1].getSizeInBits();
-      })
     .legalIf([=](const LegalityQuery &Query) {
         const LLT &Ty0 = Query.Types[0];
         const LLT &Ty1 = Query.Types[1];
         return (Ty0.getSizeInBits() % 32 == 0) &&
                (Ty1.getSizeInBits() % 32 == 0);
       });
 
   // Merge/Unmerge