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