[x86] Restructure the comments and the conditions for handling

dynamic blends.

This makes it much more clear what is going on. The case we're handling
is that of dynamic conditions, and we're bailing when the nature of the
vector types and subtarget preclude lowering the dynamic condition
vselect as an actual blend.

No functionality changed here, but this will make a subsequent bug-fix
to this code much more clear.

llvm-svn: 230690