Thumb2: Reduce memory used for fixup dependencies.

Mark fixups for far backward branches as 32-bit from the
beginning and ignore their dependencies since they cannot
be expanded anymore. This reduces the memory used by the
fixup_dependents_ when compiling methods with a lot of
slow paths.

Bug: 28256882
Change-Id: I0b7155ed8970b69703fa9c4666d9c4961e425721