Modifications to make x86 register spilling to work reasonably.  It
should work ok most of the time, although there is still a remote
possibility that the allocator simply will be unable to complete
spilling, and will just give up.

-- Incrementally try with 0, 1, 2 and 3 spill regs, so as not to
   unduly restrict the supply of regs in code which doesn't need spilling.

-- Remove the use of %ecx for shift values, so it is always available
   as the first-choice spill temporary.  For code which doesn't do
   int division, make %edx and %eax available for spilling too.
   Shifts by a non-constant amount (very rare) are now done by
   a short test-and-jump sequence, so that %ecx is not tied up.

-- x86 FP: do sin, cos, tan in-line so we get the same answers as gcc.

-- Moved a little code around to remove recursive dependencies.

-- Fix a subtle bug in x86 regUsage, which could cause underestimation
   of live ranges.