cmd/compile: fix tuple-generating flag ops as clobbering flags

cmd/compile: fix tuple-generating flag ops as clobbering flags

If an op generates a tuple, and part of that tuple is of flags type,
then treat the op as clobbering flags.

Normally this doesn't matter because we do:

v1 = ADDS        <int32, flags>
v2 = Select0 v1  <int32>
v3 = Select1 v1  <flags>

And v3 will do the right clobbering of flags.  But in the rare
cases where we issue a tuple-with-flag op and the flag portion
is dead, then we never issue a Select1.  But v1 still clobbers flags,
so we need to respect that.

Fixes builder failure in CL 28950.

Change-Id: I589089fd81aaeaaa9750bb8d85e7b10199aaa002
Reviewed-on: https://go-review.googlesource.com/29083
Reviewed-by: Cherry Zhang <cherryyz@google.com>