Fix several incorrect uses of ApplySite::getArgumentConvention.

At least most of these were latent bugs since the code was
unreachable in the PartialApply case. But that's no excuse to misuse
the API.

Also, whenever referring to an integer index, be explicit about
whether it is an applied argument or callee argument.

lib/SIL/SILVerifier.cpp

@@ -2713,7 +2713,7 @@ public:
     auto isConsumingOrMutatingApplyUse = [](Operand *use) -> bool {
       ApplySite apply(use->getUser());
       assert(apply && "Not an apply instruction kind");
-      auto conv = apply.getArgumentConvention(use->getOperandNumber() - 1);
+      auto conv = apply.getArgumentConvention(*use);
       switch (conv) {
       case SILArgumentConvention::Indirect_In_Guaranteed:
         return false;