Small structs for PPC64 SVR4 must be passed right-justified in registers.

lib/Target/PowerPC/PPCISelLowering.{h,cpp}
 Rename LowerFormalArguments_Darwin to LowerFormalArguments_Darwin_Or_64SVR4.
 Rename LowerFormalArguments_SVR4 to LowerFormalArguments_32SVR4.
 Receive small structs right-justified in LowerFormalArguments_Darwin_Or_64SVR4.
 Rename LowerCall_Darwin to LowerCall_Darwin_Or_64SVR4.
 Rename LowerCall_SVR4 to LowerCall_32SVR4.
 Pass small structs right-justified in LowerCall_Darwin_Or_64SVR4.

test/CodeGen/PowerPC/structsinregs.ll
 New test.

llvm-svn: 164228