Simpler word-sized variable right shifts on x86 and x86-64

Simpler word-sized variable right shifts on x86 and x86-64

 * Known negative shifts are converted to another function that
   only handles machine-friendly right shifts.

 * The transforms and VOPs are conditionalised on ash-right-vops,
   so other platforms aren't penalised.

 * The new transforms trigger a lot of notes; this is suboptimal,
   and one test had to be adjusted.