Relax cross-section branches

[dhoekwater]

Summary

Because the code layout is not known during compilation, the distance of
cross-section jumps is not knowable at compile-time. Unconditional branches
are relaxed via thunk insertion by the linker, but conditional branches
must be manually relaxed. Because of this, we should assume that any
cross-sectional conditional jumps are out of range. This assumption is
necessary for machine function splitting on Arm.

[dhoekwater]

Add a missing semicolon

[dhoekwater]

Add unconditional branch relaxation for AArch64 and remove the flag

[dhoekwater]

Make all getCrossSectionBranchDistance instances return uint64_t

[dhoekwater]

Add checks to tests

[dhoekwater]

Rebase updated changes and clang-format

[dhoekwater]

Clarify documentation comment wording

[arsenm]

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

271–273

I think it would be better to just fix the scavenger to tolerate empty blocks.