Reapply [InstSimplify] Support all instructions in simplifyWithOpReplaced()

Relative to the previous attempt, this includes a bailout for phi
nodes, whose arguments might refer to a previous cycle iteration.

We did not hit this before by a fortunate deficiency of the
ConstantFoldInstOperands() API, which doesn't handle phi nodes,
unlike ConstantFoldInstruction().

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Instead of hardcoding a few instruction kinds, use the generic
interface now that we have it.

The primary effect of this is that intrinsics are now supported.

It's worth noting that this is still limited in that it does not
support vectors, so we can't remove e.g. existing fshl special
cases.

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