llvm-stress: don't make vectors of x86_mmx type

LangRef.html says:
"There are no arrays, vectors or constants of this type."

This was hitting assertions when passing the -generate-x86-mmx
option.

PR12452.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154445 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed