[PPC64] Update tests for vec_sld

Revision 224297 modified the behavior of vec_sld for little endian so
that LLVM will generate the correct corresponding vsldoi instruction.
I neglected to update the existing tests, which continued to pass
because they were not specific enough.  This patch adds enough
specificity to the tests to make them useful for BE and LE testing of
vec_sld.

llvm-svn: 242313