MemoryBuiltins: getAllocAlignment is now useful for non-allocator funcs

[durin42]

Summary

This has been true since dba73135c8b7a02afb535328a7475e0a6890c271, but
didn't matter until now because clang wasn't emitting allocalign
attributes.

[nikic]

LGTM

llvm/lib/Analysis/MemoryBuiltins.cpp

bool llvm::isAllocRemovable(const CallBase *CB, const TargetLibraryInfo *TLI) {
   // expression [expr.new paragraph 13].
 
   // Historically we've treated the C family allocation routines as removable
   return isAllocLikeFn(CB, TLI);
 }
 
 Value *llvm::getAllocAlignment(const CallBase *V,
                                const TargetLibraryInfo *TLI) {
-  assert(isAllocationFn(V, TLI));
-
   const Optional<AllocFnsTy> FnData = getAllocationData(V, AnyAlloc, TLI);
   if (FnData.hasValue() && FnData->AlignParam >= 0) {
     return V->getOperand(FnData->AlignParam);
   }
   unsigned AllocAlignParam;
   if (V->getAttributes().hasAttrSomewhere(Attribute::AllocAlign,
                                           &AllocAlignParam)) {
     return V->getOperand(AllocAlignParam-1);