[InstCombine] refine select-of-constants to bitwise ops

Add logic for the special case when a cmp+select can clearly be
reduced to just a bitwise logic instruction, and remove an 
over-reaching chunk of general purpose bit magic. The primary goal 
is to remove cases where we are not improving the IR instruction 
count when doing these select transforms, and in all cases here that 
is true.

In the motivating 3-way compare tests, there are further improvements
because we can combine/propagate select values (not sure if that
belongs in instcombine, but it's there for now).

DAGCombiner has folds to turn some of these selects into bit magic,
so there should be no difference in the end result in those cases.
Not all constant combinations are handled there yet, however, so it
is possible that some targets will see more cmov/csel codegen with
this change in IR canonicalization. 

Ideally, we'll go further to *not* turn selects into multiple 
logic/math ops in instcombine, and we'll canonicalize to selects.
But we should make sure that this step does not result in regressions
first (and if it does, we should fix those in the backend).

The general direction for this change was discussed here:
http://lists.llvm.org/pipermail/llvm-dev/2016-September/105373.html
http://lists.llvm.org/pipermail/llvm-dev/2017-July/114885.html

Alive proofs for the new bit magic:
https://rise4fun.com/Alive/XG7

Differential Revision: https://reviews.llvm.org/D46086

llvm-svn: 331486

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