[SystemZ] Mark unsaved argument R6 as live throughout function.
For historical reasons, the R6 register is a callee-saved argument
register. This means that if it is used to pass an argument to a function
that does not clobber it, it is live throughout the function.
This patch makes sure that in this special case any kill flags of it are
Review: Ulrich Weigand, Eli Friedman
Differential Revision: https://reviews.llvm.org/D89451