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