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[SystemZ] Mark unsaved argument R6 as live throughout function.

Authored by jonpa on Oct 15 2020, 1:04 AM.

Description

[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
removed.

Review: Ulrich Weigand, Eli Friedman

Differential Revision: https://reviews.llvm.org/D89451

Details