[x86] Undo a flawed transform I added to form UNPCK instructions when
AVX is available, and generally tidy up things surrounding UNPCK
formation.

Originally, I was thinking that the only advantage of PSHUFD over UNPCK
instruction variants was its free copy, and otherwise we should use the
shorter encoding UNPCK instructions. This isn't right though, there is
a larger advantage of being able to fold a load into the operand of
a PSHUFD. For UNPCK, the operand *must* be in a register so it can be
the second input.

This removes the UNPCK formation in the target-specific DAG combine for
v4i32 shuffles. It also lifts the v8 and v16 cases out of the
AVX-specific check as they are potentially replacing multiple
instructions with a single instruction and so should always be valuable.
The floating point checks are simplified accordingly.

This also adjusts the formation of PSHUFD instructions to attempt to
match the shuffle mask to one which would fit an UNPCK instruction
variant. This was originally motivated to allow it to match the UNPCK
instructions in the combiner, but clearly won't now.

Eventually, we should add a MachineCombiner pass that can form UNPCK
instructions post-RA when the operand is known to be in a register and
thus there is no loss.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217755 91177308-0d34-0410-b5e6-96231b3b80d8
Fix a non-virtual destructor warning introduced in r217747.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217756 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Teach the x86 DAG combiner to form MOVSLDUP and MOVSHDUP
instructions when it finds an appropriate pattern.

These are lovely instructions, and its a shame to not use them. =] They
are fast, and can hand loads folded into their operands, etc.

I've also plumbed the comment shuffle decoding through the various
layers so that the test cases are printed nicely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217758 91177308-0d34-0410-b5e6-96231b3b80d8