[PPC64LE] Fix vec_sld semantics for little endian

The vec_sld interface provides access to the vsldoi instruction.
Unlike most of the vec_* interfaces, we do not attempt to change the
generated code for vec_sld based on the endian mode.  It is too
difficult to correctly infer the desired semantics because of
different element types, and the corrected instruction sequence is
expensive, involving loading a permute control vector and performing a
generalized permute.

For GCC, this was implemented as "Don't touch the vec_sld"
implementation.  When it came time for the LLVM implementation, I did
the same thing.  However, this was hasty and incorrect.  In LLVM's
version of altivec.h, vec_sld was previously defined in terms of the
vec_perm interface.  Because vec_perm semantics are adjusted for
little endian, this means that leaving vec_sld untouched causes it to
generate something different for LE than for BE.  Not good.

This patch adjusts the form of vec_perm that is used for vec_sld and
vec_vsldoi, effectively undoing the modifications so that the same
vsldoi instruction will be generated for both BE and LE.

There is an accompanying back-end patch to take care of some small
ripple effects caused by these changes.

llvm-svn: 242297