[PATCH][PPC64LE] Correct little-endian usage of vmrgh* and vmrgl*.

Because the PowerPC vmrgh* and vmrgl* instructions have a built-in
big-endian bias, it is necessary to swap their inputs in little-endian
mode when using them to implement a vector shuffle.  This was
previously missed in the vector LE implementation.

There was already logic to distinguish between unary and "normal"
vmrg* vector shuffles, so this patch extends that logic to use a third
option:  "swapped" vmrg* vector shuffles that are used for little
endian in place of the "normal" ones.

I've updated the vec-shuffle-le.ll test to check for the expected
register ordering on the generated instructions.

This bug was discovered when testing the LE and ELFv2 patches for
safety if they were backported to 3.4.  A different vectorization
decision was made in 3.4 than on mainline trunk, and that exposed the
problem.  I've verified this fix takes care of that issue.