[RegisterScavenging] Respect early-clobber defs when scavenging registers (#197120) When scavenging registers backwards for virtual registers introduced during frame index elimination, the register scavenger was ignoring early-clobber constraints on the instruction using the scavenged register. This could lead to assigning a virtual register to a physical register marked as early-clobber output, violating the constraint that early-clobber outputs cannot overlap with inputs. This change inspects `RestoreAfter` to determine if the scavenged register will be used by the instruction pointed at by MBBI, and if so, remove any such registers from the scavengeable set. This also adds a test to check if such EC defs are indeed respected whne they otherwise wouldn't be. co-authored-by: @uweigand --------- Co-authored-by: Matt Arsenault <arsenm2@gmail.com>
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