When `simplifyBrToBlockWithSinglePred` merges a block into its sole
predecessor, it calls `inlineBlockBefore` which replaces each block
argument with the corresponding value passed by the branch. If one of
those values is itself a block argument of the successor block, the call
`replaceAllUsesWith(arg, arg)` is a no-op. Any uses of that argument
outside the block (e.g. in a downstream block) are therefore not
replaced, and when the successor block is erased the argument is
destroyed while those uses are still live, triggering the assertion
`use_empty() && "Cannot destroy a value that still has uses\!"` in
`IRObjectWithUseList::~IRObjectWithUseList`.
Guard against this by returning early when any branch operand is a block
argument owned by the destination block.
Fixes#126213
1073951bdb