Three microoptimizations to numeric tag testing for 1-bit fixnum tag.

Each of the following might save 1 byte on x86-64, depending on
register allocation.

- Instead of "LEA EAX, [Rxx - other-pointer-lowtag]" use full-width RAX.
  At worst this adds a REX prefix, but might not if Rxx needed it anyway.
  The payback is in loading widetag from BYTE PTR [RAX].
  After the LEA we never need a displacement, which either breaks even
  by saving 1 byte, or wins - nets a 1 byte savings if Rxx had required
  a REX prefix.
  Note: RAX is an alias pointer, but safe because of GC conservativism.

These two depend on AL having the shortest encoding:
- Do the fixnum lowtag test only after loading RAX by checking for
  a 1 bit instead of looking for a 0 bit in the original value.
- Similarly for SHORT-FLOAT, do the immediate tag test on AL
  compensating for the biased value in the widetag field.