[X86] Check MI.isConvertibleTo3Addr() before calling
convertToThreeAddress in X86FixupLEAs.

X86FixupLEAs just assumes convertToThreeAddress will return nullptr
for any instruction that isn't convertible.

But the code in convertToThreeAddress for X86 assumes that any
instruction coming in has at least 2 operands and that the second one
is a register. But those properties aren't guaranteed of all
instructions. We should check the instruction property first.

[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.