[PPC64LE] Enable missing lxvdsx optimization, and related swap optimization

When adding little-endian vector support for PowerPC last year, I
inadvertently disabled an optimization that recognizes a load-splat
idiom and generates the lxvdsx instruction.  This patch moves the
offending logic so lxvdsx is once again generated.

This pattern is frequently generated by the vectorizer for scalar
loads of an effective constant.  Previously the lxvdsx instruction was
wrongly listed as lane-sensitive for the VSX swap optimization (since
both doublewords are identical, swaps are safe).  This patch fixes
this as well, so that vectorized code using lxvdsx can now have swaps
removed from the computation.

There is an existing test (@test50) in test/CodeGen/PowerPC/vsx.ll
that checks for the missing optimization.  However, vsx.ll was only
being tested for POWER7 with big-endian code generation.  I've added
a little-endian RUN statement and expected LE code generation for all
the tests in vsx.ll to give us a bit better VSX coverage, including
what's needed for this patch.

llvm-svn: 241183