DAG: Avoid bitcast/ext/build_vector combine

This avoids regressions in a future AMDGPU change
to make v4i16/v4f16 legal. For these types, build_vector
is implemented as bitcasted operations on v2i32. This
combine was creating v4i16s out of what would have been
already been a v2i32 build_vector, creating a mess
of nodes that never get cleaned up.

I'm not sure this is the right condition to check.
I initially tried just checking for the legality of the
new build_vector. This works for my case, but breaks dozens
of x86 tests. A Mips test seems to show some improvement
or at least a neutral change. I don't want to think
about how long it would take to analyze the set of
different x86 vector operations impacted.

Test included in future commit.

llvm-svn: 334218

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -14918,7 +14918,10 @@ SDValue DAGCombiner::reduceBuildVecExtToExtBuildVec(SDNode *N) {
   assert(VecVT.getSizeInBits() == VT.getSizeInBits() &&
          "Invalid vector size");
   // Check if the new vector type is legal.
-  if (!isTypeLegal(VecVT)) return SDValue();
+  if (!isTypeLegal(VecVT) ||
+      (!TLI.isOperationLegal(ISD::BUILD_VECTOR, VecVT) &&
+       TLI.isOperationLegal(ISD::BUILD_VECTOR, VT)))
+    return SDValue();
 
   // Make the new BUILD_VECTOR.
   SDValue BV = DAG.getBuildVector(VecVT, DL, Ops);