[PowerPC] Implement the vpopcnt instructions for POWER8

Patch by Kit Barton.

Add the vector population count instructions for byte, halfword, word,
and doubleword sizes.  There are two major changes here:

    PPCISelLowering.cpp: Make CTPOP legal for vector types.
    PPCRegisterInfo.td: Added v2i64 to the VRRC register
      definition. This is needed for the doubleword variations of the
      integer ops that were added in P8. 

Test Plan

Test the instruction vpcnt* encoding/decoding in ppc64-encoding-vmx.s

Test the generation of the vpopcnt instructions for various vector
data types.  When adding the v2i64 type to the Vector Register set, I
also needed to add the appropriate bit conversion patterns between
v2i64 and the existing vector types.  Testing for these conversions
were also added in the test case by passing a different vector type as
a parameter into the test functions.  There is also a run step that
will ensure the vpopcnt instructions are generated when the vsx
feature is disabled.

llvm-svn: 228046
diff --git a/llvm/lib/Target/PowerPC/PPCSubtarget.h b/llvm/lib/Target/PowerPC/PPCSubtarget.h
index 82d6587..7f674a7 100644
--- a/llvm/lib/Target/PowerPC/PPCSubtarget.h
+++ b/llvm/lib/Target/PowerPC/PPCSubtarget.h
@@ -89,6 +89,7 @@
   bool HasQPX;
   bool HasVSX;
   bool HasP8Vector;
+  bool HasP8Altivec;
   bool HasFCPSGN;
   bool HasFSQRT;
   bool HasFRE, HasFRES, HasFRSQRTE, HasFRSQRTES;
@@ -216,6 +217,7 @@
   bool hasQPX() const { return HasQPX; }
   bool hasVSX() const { return HasVSX; }
   bool hasP8Vector() const { return HasP8Vector; }
+  bool hasP8Altivec() const { return HasP8Altivec; }
   bool hasMFOCRF() const { return HasMFOCRF; }
   bool hasISEL() const { return HasISEL; }
   bool hasPOPCNTD() const { return HasPOPCNTD; }