[llvm] r243308 - [AArch64] Remove check for Darwin that was needed to decide if x18 should

[Tim Northover via llvm-commits]

> I think the right thing to do is to default ReserveX18 in the
> AArch64Subtarget constructor based on isOSDarwin. i.e.:

I think I agree. The triple ought to be enough to give reasonably ABI-compliant code, otherwise why include the OS there in the first place?

> If we do this we can also revert r243310 since it'll become redundant. WDYT?

Probably a good idea too.

Tim.