Fix backward branching in SwitchImm.

Summary:
`SwitchImm` can subtract from `ip`, but it was reading the offset
from a `uint32_t *` instead of `int32_t *`.
Change the type to the correct one.

Reviewed By: dulinriley

Differential Revision: D23114516

fbshipit-source-id: d4ddab94eb16dc1ce479590851eeb1ab428bd5d6