st/nine: Ignore D3DSIO_RET if it is the last instruction in a shader

RET as a last instruction could be safely ignored.
Remove it to prevent crashes/warnings in case underlying driver
doesn't implement arbitrary returns.

A better way would be to remove the RET after the whole shader
is parsed which will handle a possible case when the last RET is
followed by a comment.

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Axel Davy <davyaxel0@gmail.com>
40 jobs for !1917 with fix/tgsi-to-nir-ignore-ret in 36 minutes and 42 seconds (queued for 16 seconds)
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