[CostModel][X86] Tidyup arithmetic costs code. NFCI.

Remove unnecessary braces, remove one use variables and keep LUTs to similar naming convention.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291187 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed