Use ICmpInst::compare() where possible, ConstantFoldCompareInstOperands
in other places. This only changes places where the either the fold is
guaranteed to succeed, or the code doesn't use the resulting compare if
we fail to fold.

phi-known.ll:test2 required deleting one manual check for a bitcast

Also strip trailing whitespace while we're touching everything.

I noticed that we were not folding expressions like this:
icmp ult (constexpr), null
in https://llvm.org/PR49355, so we end up with extremely large
icmp instructions as the constant expressions pile up on each other.

There is no potential to mis-fold an unsigned boundary condition
with a zero/null, so this is just falling through a crack in the
pattern matching.

The more general case of comparisons of non-zero constants and
constexpr are more tricky and may require the datalayout to know
how to cast to different types, etc. Negative tests verify that
we are only changing a subset of potential patterns.

Differential Revision: https://reviews.llvm.org/D98150

See D98150.