[PowerPC] Improvements for BUILD_VECTOR Vol. 2

This patch corresponds to review:
https://reviews.llvm.org/D25980

This is the 2nd patch in a series of 4 that improve the lowering and combining
for BUILD_VECTOR nodes on PowerPC. This particular patch combines a build vector
of fp-to-int conversions into an fp-to-int conversion of a build vector of fp
values. For example:
Converts (build_vector (fp_to_[su]i $A), (fp_to_[su]i $B), ...)
Into (fp_to_[su]i (build_vector $A, $B, ...))).
Which is a natural match for much cleaner code.

llvm-svn: 288218
diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.h b/llvm/lib/Target/PowerPC/PPCISelLowering.h
index 1723d12..95f32f5 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.h
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.h
@@ -990,6 +990,10 @@
     unsigned combineRepeatedFPDivisors() const override;
 
     CCAssignFn *useFastISelCCs(unsigned Flag) const;
+
+    SDValue
+      combineElementTruncationToVectorTruncation(SDNode *N,
+                                                 DAGCombinerInfo &DCI) const;
   };
 
   namespace PPC {