Reapply [InstCombine] Switch foldOpIntoPhi() to use InstSimplify

Reapply [InstCombine] Switch foldOpIntoPhi() to use InstSimplify

Relative to the previous attempt, this is rebased over the
InstSimplify fix in ac74e7a7806480a000c9a3502405c3dedd8810de,
which addresses the miscompile reported in PR58401.

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foldOpIntoPhi() currently only folds operations into the phi if all
but one operands constant-fold. The two exceptions to this are freeze
and select, where we allow more general simplification.

This patch makes foldOpIntoPhi() generally simplification based and
removes all the instruction-specific logic. We just try to simplify
the instruction for each operand, and for the (potentially) one
non-simplified operand, we move it into the new block with adjusted
operands.

This fixes https://github.com/llvm/llvm-project/issues/57448, which
was my original motivation for the change.

Differential Revision: https://reviews.llvm.org/D134954