[X86] Don't convert 8 or 16 bit ADDs to LEAs on Atom in FixupLEAPass.

We use the functions that convert to three address to do the
conversion, but changing an 8 or 16 bit will cause it to create
a virtual register. This can't be done after register allocation
where this pass runs.

I've switched the pass completely to a white list of instructions
that can be converted to LEA instead of a blacklist that was
incorrect. This will avoid surprises if we enhance the three
address conversion function to include additional instructions
in the future.

Fixes PR42565.

llvm-svn: 365720