[x86] Hoist the logic for extracting the relevant bits of information
from the MachineInstr into the caller which is already doing a switch
over the instruction.

This will make it more clear how to compute different operands to feed
the comment selection for example.

Also, in a drive-by-fix, don't append an empty comment string (which is
a no-op ultimately).

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218361 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Bypass the shuffle mask comment generation when not using verbose
asm. This can be somewhat expensive and there is no reason to do it
outside of tests or debugging sessions. I'm also likely to make it
significantly more expensive to support more styles of shuffles.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218362 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] More refactoring of the shuffle comment emission. The previous
attempt didn't work out so well. It looks like it will be much better
for introducing extra logic to find a shuffle mask if the finding logic
is totally separate. This also makes it easy to sink the opcode logic
completely out of the routine so we don't re-dispatch across it.

Still no functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218363 91177308-0d34-0410-b5e6-96231b3b80d8