[RS4GC] Remove a redundant linear search, NFCI

Since LiveVariables is uniqued (we just created it from a `DenseSet`),
`FindIndex(LiveVariables, LiveVariables[i])` is always `i`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250786 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed