So ARM had quite a few codegen bugs on LLVM 4.0 which are fixed on LLVM
trunk. This backports 5 of them:
r297871 - ARM: avoid clobbering register in v6 jump-table expansion.
- fixesrust-lang/rust#42248
r294949 - [Thumb-1] TBB generation: spot redefinitions of index
r295816 - [ARM] Fix constant islands pass.
r300870 - [Thumb-1] Fix corner cases for compressed jump tables
r302650 - [IfConversion] Add missing check in
IfConversion/canFallThroughTo
- unblocks rust-lang/rust#39409