- dynamic allocations could generate
  bad 'and' instructions (for the
  and with -16 in salloc()).
- symbols used in w context would
  generate adrp and add instructions
  on wN registers while they seem to
  only work on xN registers.

Thanks to Rosie for reporting them.

When rbp is not necessary to compile
a leaf function, we skip saving and
restoring it.

Clang incorrectly optimizes this negation with -O2 and causes QBE to
emit 0 in place of INT64_MIN.

Functions are now aligned on 16-byte
boundaries. This mimics gcc and should
help reduce the maximum perf impact of
cosmetic code changes. Previously, any
change in the output of qbe could have
far reaching implications on alignment.

Thanks to Roland Paterson-Jones for
pointing out the variability issue.

This was cute to do, but it is
largely inconsequential, as shown
by the rough timings below:

    benchmarking mul8_lea
      3.9 ticks ± 0.88 (min: 3)
    benchmarking mul8_imul
      3.3 ticks ± 0.27 (min: 3)
    benchmarking div8_udiv
      6.5 ticks ± 0.52 (min: 6)
    benchmarking div8_shr
      3.3 ticks ± 0.34 (min: 3)

Additionally, the strength-reduction
for small powers of two is handled
by amd64/emit.c now.