[InstCombine] safely allow non-commutative binop identity constant folds

This was originally intended with D48893, but as discussed there, we
have to make the folds safe from producing extra poison. This should
give the single binop folds the same capabilities as the existing
folds for 2-binops+shuffle.

LLVM binary opcode review: there are a total of 18 binops. There are 7 
commutative binops (add, mul, and, or, xor, fadd, fmul) which we already 
fold. We're able to fold 6 more opcodes with this patch (shl, lshr, ashr,
fdiv, udiv, sdiv). There are no folds for srem/urem/frem AFAIK. We don't 
bother with sub/fsub with constant operand 1 because those are 
canonicalized to add/fadd. 7 + 6 + 3 + 2 = 18.

llvm-svn: 336684