The x18 is a reserved platform register on Darwin and Windows.

x8/w8 seems to be unused in this function though (and same about
x10 and x14), so there's really no reason to use x18 here - just change
the uses of x18/w18 into x8/w8 instead without any further rewrites.
Signed-off-by: Martin Storsjö <martin@martin.st>