[AArch64][GlobalISel] NFC: Remove dead G_BUILD_VECTOR legalization rule

[paquette]

Summary

Remove a rule which allows larger scalar types than the destination vector element type.

This appears to be irrelevant now that we have G_BUILD_VECTOR_TRUNC. Plus, making a G_BUILD_VECTOR which satisfies this introduces a verifier failure anyway.

[aemerson]

LGTM.

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp

getActionDefinitionsBuilder(G_BUILD_VECTOR)
                  {v4s16, s16},
                  {v8s16, s16},
                  {v2s32, s32},
                  {v4s32, s32},
                  {v2p0, p0},
                  {v2s64, s64}})
       .clampNumElements(0, v4s32, v4s32)
       .clampNumElements(0, v2s64, v2s64)
-
-      // Deal with larger scalar types, which will be implicitly truncated.
-      .legalIf([=](const LegalityQuery &Query) {
-        return Query.Types[0].getScalarSizeInBits() <
-               Query.Types[1].getSizeInBits();
-      })
       .minScalarSameAs(1, 0);
 
   getActionDefinitionsBuilder(G_CTLZ)
       .legalForCartesianProduct(
           {s32, s64, v8s8, v16s8, v4s16, v8s16, v2s32, v4s32})
       .scalarize(1);
 
   getActionDefinitionsBuilder(G_SHUFFLE_VECTOR)