[x86] Make the x86 PACKSSWB, PACKSSDW, PACKUSWB, and PACKUSDW
instructions available as synthetic SDNodes PACKSS and PACKUS that will
select to the correct instruction variants based on the return type.
This allows us to use these rather important instructions when lowering
vector shuffles.

Also moves the relevant instruction definitions to be split out from
the fully generic multiclasses to allow them to match these new SDNodes
in the same way that the UNPCK instructions do.

No functionality should actually be changed here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211332 91177308-0d34-0410-b5e6-96231b3b80d8
Support: Write ScaledNumbers::getRounded()

Start extracting helper functions out of -block-freq's `UnsignedFloat`
into `Support/ScaledNumber.h` with the eventual goal of moving and
renaming the class to `ScaledNumber`.

The bike shed about names is still being painted, but I'm going with
this for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211333 91177308-0d34-0410-b5e6-96231b3b80d8
Fix .cpp files claiming to be header files

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211334 91177308-0d34-0410-b5e6-96231b3b80d8
Small clanups:

Use static instead of anonymous namespace.
Delete write only variables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211335 91177308-0d34-0410-b5e6-96231b3b80d8