Thumb2: Reduce memory used for fixup dependencies.

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

This is a cherry-pick of the fixed revert-revert
    Revert "Revert "Thumb2: Reduce memory used for fixup dependencies.""
    (cherry picked from commit a64f249add4a0a9e93aa6f246045ca79f5916dcd)

Change-Id: I9ca766b73d9f5a1da5dc10ad070145ac88fbcd53