Change AArch64 i128 returns to use [2 x i64] when possible.

Something like { void*, void * } would be passed to a function as a [2 x i64], but returned as an i128.  This patch unifies the 2 behaviours so that we also return it as a [2 x i64].

This is better for the quality of the IR, and the size of the final LLVM binary as we tend to want to insert/extract values from these types and do so with the insert/extract instructions is less IR than shifting, truncating, and or'ing values.

Reviewed by Tim Northover.

llvm-svn: 235231