[ValueTracking] Short-circuit computeKnownBitsAddSub(); NFCI

If one operand is unknown (and we don't have nowrap), don't compute
the second operand.

Also don't create an unnecessary extra KnownBits variable, it's
okay to reuse KnownOut.

This reduces instructions on libclamav_md5.c by 40%.

llvm/lib/Analysis/ValueTracking.cpp

@@ -385,15 +385,15 @@ static void computeKnownBitsAddSub(bool Add, const Value *Op0, const Value *Op1,
                                    bool NSW, const APInt &DemandedElts,
                                    KnownBits &KnownOut, KnownBits &Known2,
                                    unsigned Depth, const Query &Q) {
-  unsigned BitWidth = KnownOut.getBitWidth();
+  computeKnownBits(Op1, DemandedElts, KnownOut, Depth + 1, Q);
 
-  // If an initial sequence of bits in the result is not needed, the
-  // corresponding bits in the operands are not needed.
-  KnownBits LHSKnown(BitWidth);
-  computeKnownBits(Op0, DemandedElts, LHSKnown, Depth + 1, Q);
-  computeKnownBits(Op1, DemandedElts, Known2, Depth + 1, Q);
+  // If one operand is unknown and we have no nowrap information,
+  // the result will be unknown independently of the second operand.
+  if (KnownOut.isUnknown() && !NSW)
+    return;
 
-  KnownOut = KnownBits::computeForAddSub(Add, NSW, LHSKnown, Known2);
+  computeKnownBits(Op0, DemandedElts, Known2, Depth + 1, Q);
+  KnownOut = KnownBits::computeForAddSub(Add, NSW, Known2, KnownOut);
 }
 
 static void computeKnownBitsMul(const Value *Op0, const Value *Op1, bool NSW,