[mlir] Fix -Wsign-compare in ValueBoundsOpInterface.cpp (NFC)

/llvm-project/mlir/lib/Interfaces/ValueBoundsOpInterface.cpp:762:16:
error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
        rhsPos >= cstr.positionToValueDim.size())
        ~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/llvm-project/mlir/lib/Interfaces/ValueBoundsOpInterface.cpp:761:16:
error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
    if (lhsPos >= cstr.positionToValueDim.size() ||
        ~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

Author: DamonFool

mlir/lib/Interfaces/ValueBoundsOpInterface.cpp

@@ -758,8 +758,8 @@ bool ValueBoundsConstraintSet::compare(AffineMap lhs, ValueDimList lhsOperands,
   auto stopCondition = [&](Value v, std::optional<int64_t> dim,
                            ValueBoundsConstraintSet &cstr) {
     // Keep processing as long as lhs/rhs were not processed.
-    if (lhsPos >= cstr.positionToValueDim.size() ||
-        rhsPos >= cstr.positionToValueDim.size())
+    if (size_t(lhsPos) >= cstr.positionToValueDim.size() ||
+        size_t(rhsPos) >= cstr.positionToValueDim.size())
       return false;
     // Keep processing as long as the relation cannot be proven.
     return cstr.comparePos(lhsPos, cmp, rhsPos);