transform: fix error in interface lowering pass

It appears that LLVM can sometimes recognize that multiple calls to
runtime.interfaceMethod can be merged into one. When that happens, the
interface lowering pass shows an error as it didn't expect that
situation.

Luckily the fix is very easy.

transform/interface-lowering.go

@@ -317,11 +317,7 @@ func (p *lowerInterfacesPass) run() error {
 			}
 			inttoptr := inttoptrs[0]
 			calls := getUses(inttoptr)
-			if len(calls) != 1 || calls[0].IsACallInst().IsNil() {
-				return errorAt(use, "internal error: expected exactly one call use of runtime.interfaceMethod")
-			}
-			call := calls[0]
-
+			for _, call := range calls {
 				// Set up parameters for the call. First copy the regular params...
 				params := make([]llvm.Value, call.OperandsCount())
 				paramTypes := make([]llvm.Type, len(params))
@@ -344,6 +340,7 @@ func (p *lowerInterfacesPass) run() error {
 					call.ReplaceAllUsesWith(retval)
 				}
 				call.EraseFromParentAsInstruction()
+			}
 			inttoptr.EraseFromParentAsInstruction()
 			use.EraseFromParentAsInstruction()
 		}