[SROA] Use a more direct way of determining whether we are processing

the destination operand or source operand of a memmove.

It so happens that it was impossible for SROA to try to rewrite
self-memmove where the operands are *identical*, because either such
a think is volatile (and we don't rewrite) or it is non-volatile, and we
don't even register it as a use of the alloca.

However, making the 'IsDest' test *rely* on this subtle fact is... Very
confusing for the reader. We should use the direct and readily available
test of the Use* which gives us concrete information about which operand
is being rewritten.

No functionality changed, I hope! ;]

llvm-svn: 202103

lib/Transforms/Scalar/SROA.cpp

@@ -2428,8 +2428,9 @@ private:
     uint64_t NewBeginOffset = std::max(BeginOffset, NewAllocaBeginOffset);
     uint64_t NewEndOffset = std::min(EndOffset, NewAllocaEndOffset);
 
-    assert(II.getRawSource() == OldPtr || II.getRawDest() == OldPtr);
-    bool IsDest = II.getRawDest() == OldPtr;
+    bool IsDest = &II.getRawDestUse() == OldUse;
+    assert(IsDest && II.getRawDest() == OldPtr ||
+           (!IsDest && II.getRawSource() == OldPtr));
 
     // Compute the relative offset within the transfer.
     unsigned IntPtrWidth = DL.getPointerSizeInBits();