[PPC64LE] Recognize shufflevector patterns for little endian

Various masks on shufflevector instructions are recognizable as
specific PowerPC instructions (vector pack, vector merge, etc.).
There is existing code in PPCISelLowering.cpp to recognize the correct
patterns for big endian code.  The masks for these instructions are
different for little endian code due to the big-endian numbering
employed by these instructions.  This patch adds the recognition code
for little endian.

I've added a new test case test/CodeGen/PowerPC/vec_shuffle_le.ll for
this.  The existing recognizer test (vec_shuffle.ll) is unnecessarily
verbose and difficult to read, so I felt it was better to add a new
test rather than modify the old one.