FuncOp::eraseBody: drop all references before erasing blocks

Operations in a block can use a value defined in a dominating block.  When a
block, and therefore all its operations, is deleted, the operations are not
allowed to have any remaining uses.  Drop all uses of values in all blocks
before deleting them in FuncOp::eraseBody to avoid deleting an operation before
deleting the users of its results.

PiperOrigin-RevId: 257628002