Make APInt negate just do a 2's complement negate instead of subtract. NFC.

This is part of an effort to shave allocations from APInt heavy paths.  I'll
be moving many of the other operators to r-value references soon and this is
a step towards doing that without too much duplication.

Saves 15k allocations when doing 'opt -O2 verify-uselistorder.bc'.

llvm-svn: 271556

llvm/include/llvm/ADT/APInt.h

@@ -625,7 +625,12 @@ public:
   /// Negates *this using two's complement logic.
   ///
   /// \returns An APInt value representing the negation of *this.
-  APInt operator-() const { return APInt(BitWidth, 0) - (*this); }
+  APInt operator-() const {
+    APInt Result(*this);
+    Result.flipAllBits();
+    ++Result;
+    return Result;
+  }
 
   /// \brief Logical negation operator.
   ///