[CFIFixup] Factor logic into helpers and use range-based loops (NFC)

[dhoekwater]

runOnMachineFunction is getting long (>100 lines), and the logic
for computing block info and performing block fixup can be abstracted
away.

Reduce nesting in the main block fixup loop and name conditions to
reflect their purpose.

Replace manual usage of iterators with a range-based for loop.

Source:

• https://llvm.org/docs/CodingStandards.html#use-early-exits-and-continue-to-simplify-code
• https://llvm.org/docs/CodingStandards.html#use-range-based-for-loops-wherever-possible
• https://llvm.org/docs/CodingStandards.html#don-t-evaluate-end-every-time-through-a-loop

[kazutakahirata]

LGTM. Thanks!