[llvm] r228518 - ARM & AArch64: teach LowerVSETCC that output type size may differ from input.

[Tim Northover]

>> Should fix PR21645.
>
> Is this suitable for merging to 3.6?

I think so, the changes are fairly obvious and we've got at least one
user who's encountered the problem in the wild.

Cheers.

Tim.