No longer check for IceInst sources being NULL.

Now that optimized address mode memory operands are properly
represented instead of converted to a stream of 4 operands, there is
no more need for code that checks whether a source operand is NULL.
Remove the old findAddressOpt/doAddressOpt code.

This has all been pushed into the IceTargetLowering layer, and the
original Vanilla ICE code is no longer needed.
Reformat using "make format".