[InstCombine] fix operand-complexity-based canonicalization (PR28296)

The code comments didn't match the code logic, and we didn't actually distinguish the fake unary (not/neg/fneg) 
operators from arguments. Adding another level to the weighting scheme provides more structure and can help 
simplify the pattern matching in InstCombine and other places.

I fixed regressions that would have shown up from this change in:
rL290067
rL290127

But that doesn't mean there are no pattern-matching logic holes left; some combines may just be missing regression tests.

Should fix:
https://llvm.org/bugs/show_bug.cgi?id=28296

Differential Revision: https://reviews.llvm.org/D27933

llvm-svn: 294049