Do not assume Blocks belong to Functions

    Fix Block::splitBlock and Block::eraseFromFunction that erronously assume
    blocks belong to functions.  They now belong to regions.  When splitting, new
    blocks should be created in the same region as the existing block.  When
    erasing a block, it should be removed from the region rather than from the
    function body that transitively contains the region.

    Also rename Block::eraseFromFunction to Block::erase for consistency with other
    IR containers.

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PiperOrigin-RevId: 250278272