[InstCombine] add tests for extract of vector load; NFC

There's a mismatch internally about how we are handling these patterns. 
We count loads as cheapToScalarize(), but then we don't actually 
scalarize them, so that can leave extra instructions compared to where 
we started when scalarizing other ops. If it's cheapToScalarize, then 
we should be scalarizing.

llvm-svn: 349560