[X86] Added LSHR cost model tests

There are several dodgy costings due to AVX1 legalizing 256-bit integer vectors that need fixing.

As discussed in D8690. 

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249983 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed