There is a mechanism to avoid redundant `as` ZIR instructions which is
to pass `ResultLoc.coerced_ty` instead of `ResultLoc.ty` when it is
known by AstGen that Sema will do the coercion.

This commit downgrades `coerced_ty` to `ty` when a result location
passes through an expression that branches, such as `if`, `switch`,
`while`, and `for`, causing the `as` ZIR instruction to be emitted.

This ensures that the type of a result location will be applied to, e.g.
a `comptime_int` on either side of a branch on a runtime condition.