Update inselt tests at llvm/test/Analysis to have poison as shufflevector's placeholder (NFC)

File listed by:

grep -R -E "^[^;]*shufflevector <.*> .*, <.*> undef" . | grep inseltpoison

Updated with:

sed -i -E 's/shufflevector <(.*)> (.*), <(.*)> undef/shufflevector <\1> \2, <\3> poison/g' $1