[MemCpyOpt] Use doesNotCapture() helper (NFC)

No difference in semantics here as byval is already handled
separately. This simplifies migration to the captures attribute.

Author: nikic

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -2010,7 +2010,7 @@ bool MemCpyOptPass::processImmutArgument(CallBase &CB, unsigned ArgNo) {
   Value *ImmutArg = CB.getArgOperand(ArgNo);
 
   // 1. Ensure passed argument is immutable during call.
-  if (!CB.paramHasAttr(ArgNo, Attribute::NoCapture))
+  if (!CB.doesNotCapture(ArgNo))
     return false;
 
   // We know that the argument is readonly at this point, but the function