Don't allocate in APInt::slt. NFC.

APInt::slt was copying the LHS and RHS in to temporaries then making
them unsigned so that it could use an unsigned comparision.  It did
this even on the paths which were trivial to give results for, such
as the sign bit of the LHS being set while RHS was not set.

This changes the logic to return out immediately in the trivial cases,
and use an unsigned comparison in the remaining cases.  But this time,
just use the unsigned comparison directly without creating any temporaries.

This works because, for example:
  true = (-2 slt -1) = (0xFE ult 0xFF)

Also added some tests explicitly for slt with APInt's larger than 64-bits
so that this new code is tested.

Using the memory for 'opt -O2 verify-uselistorder.lto.opt.bc -o opt.bc'
(see r236629 for details), this reduces the number of allocations from
26.8M to 23.9M.

llvm-svn: 270881