patch applied (ghc): Fix a bug in optimising division to shift right

[Simon Marlow]

Wed Apr 12 07:42:47 PDT 2006  Simon Marlow <simonmar at microsoft.com>
  * Fix a bug in optimising division to shift right
  Division by an integral log2 can't be directly optimised to a shift
  right, because shift right behaves like a division that rounds to
  negative infinity, whereas we want one that rounds to zero.  Fix this
  by adding (divisor-1) to the dividend when it is negative before
  shifting.  We do this without jumps, generating very slightly worse
  code than gcc, which uses conditional moves on CPUs that support it.

    M ./compiler/cmm/CmmOpt.hs -7 +30