[InstCombine] canonicalize add/sub with bool

add A, sext(B) --> sub A, zext(B)

We have to choose 1 of these forms, so I'm opting for the
zext because that's easier for value tracking.

The backend should be prepared for this change after:
D57401
rL353433

This is also a preliminary step towards reducing the amount
of bit hackery that we do in IR to optimize icmp/select.
That should be waiting to happen at a later optimization stage.

The seeming regression in the fuzzer test was discussed in:
D58359

We were only managing that fold in instcombine by luck, and
other passes should be able to deal with that better anyway.

llvm-svn: 354748

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