[GlobalISel] Remove an unnecessary cast (NFC) (#146249)

Idx is already of unsigned.

Author: kazutakahirata

llvm/lib/CodeGen/GlobalISel/LoadStoreOpt.cpp

@@ -458,7 +458,7 @@ bool LoadStoreOpt::processMergeCandidate(StoreMergeCandidate &C) {
     for (auto AliasInfo : reverse(C.PotentialAliases)) {
       MachineInstr *PotentialAliasOp = AliasInfo.first;
       unsigned PreCheckedIdx = AliasInfo.second;
-      if (static_cast<unsigned>(Idx) < PreCheckedIdx) {
+      if (Idx < PreCheckedIdx) {
         // Once our store index is lower than the index associated with the
         // potential alias, we know that we've already checked for this alias
         // and all of the earlier potential aliases too.