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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@271556 91177308-0d34-0410-b5e6-96231b3b80d8

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