target-ppc: add vrldnmi and vrlwmi instructions

vrldmi: Vector Rotate Left Dword then Mask Insert
vrlwmi: Vector Rotate Left Word then Mask Insert

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
( use extract[32,64] and rol[32,64], introduce mask helpers in
  internal.h )
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>