[SparseSolver] Rename getLatticeState to getExistingValueState (NFC)

The new name clarifies the function's relation to getValueState. That is,
unlike getValueState, the state of a given value will not be initialized if
it's not already in the map.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315356 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed