[x86] Fix another miscompile in the new vector shuffle lowering found

[x86] Fix another miscompile in the new vector shuffle lowering found
through the new fuzzer.

This one is great: bad operator precedence led the modulus to happen at
the wrong point. All the asserts didn't fire because there were usually
the right values past the end of the 4 element region we were looking
at. Probably could have gotten a crash here with ASan + fuzzing, but the
correctness tests pinpointed this really nicely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215092 91177308-0d34-0410-b5e6-96231b3b80d8