[PPC64LE] Fix vec_sld semantics for little endian

[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 back-end patch accompanies the changes to altivec.h that change
vec_sld's behavior for little endian.  Those changes mean that we see
slightly different code in the back end when trying to recognize a
VSLDOI instruction in isVSLDOIShuffleMask.  In particular, a
ShuffleKind of 1 (where the two inputs are identical) must now be
treated the same way as a ShuffleKind of 2 (little endian with
different inputs) when little endian mode is in force.  This is
because ShuffleKind of 1 is defined using big-endian numbering.

This has a ripple effect on LowerBUILD_VECTOR, where we create our own
internal VSLDOI instructions.  Because these are a ShuffleKind of 1,
they will now have their shift amounts subtracted from 16 when
recognizing the shuffle mask.  To avoid problems we have to subtract
them from 16 again before creating the VSLDOI instructions.

There are a couple of other uses of BuildVSLDOI, but these do not need
to be modified because the shift amount is 8, which is unchanged when
subtracted from 16.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242296 91177308-0d34-0410-b5e6-96231b3b80d8