[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