AArch64: share slgihtly more NEON implementation with ARM.

The s64/u64 vcvt conversion operations are actually pretty much identical to
the s32/u32 ones in implementation, and can be shared with just one extra
variable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201145 91177308-0d34-0410-b5e6-96231b3b80d8