[x86] limit extractelement of setcc to pre-legalization

A fuzzer found the crasher:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13700

The bug was introduced recently here:
rL355741

This is the quick fix. If we need to do this transform
later, then we'd have to extend/truncate the vector setcc
element type to the scalar setcc type (i8). 

llvm-svn: 356053