inlining: use method match signature for union-spliting (#53600)
In cases where the results of constant inference, like concrete-eval,
are used for union-split inlining, `isa`-blocks are generated using the
`result.edge.specTypes` stored within each `result`. However, it's been
found that the `edge` returned by abstract interpretation may have been
widened by the new `@nospecializeinfer`, which can result in invalid
union-splitting. To address this problem, this commit tweaks the
inlining algorithm so that it performs union-split inlining using the
original signatures that abstract interpretation used for union-split
inference, by using `match::MethodMatch`.
- fixes #53590
