[AArch64][v8.3a] Add missing imp-defs on RETA*.

RETA always implicitly uses LR, unlike RET which merely has an
alias that defaults it to LR.
Additionally, RETA implicitly uses SP as well, which it uses as
a discriminator to authenticate LR.

This isn't usually noticeable, because RET_ReallyLR is used in most
of the backend.  However, the post-RA scheduler, if enabled, will
cause miscompiles if the imp-uses are missing.

While there, fix a typo in the lone affected testcase.