[x86] Fix a few typos in my comments spotted in passing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214626 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Switch to using the variable we extracted this operand into.

Spotted this missed refactoring by inspection when reading code, and it
doesn't changethe functionality at all.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214627 91177308-0d34-0410-b5e6-96231b3b80d8