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JIT: fix potential problem in transition to 64-bit jumps

On x86_64, the JIT compiler initially generates code with 32-bit
jumps, but it switches to 64-bit jumps when so much code is allocated
that it gets spaced out enough. That transition could happen during a
recursive call to the JIT compiler or while one place is in the JIT
and other installs a shared code pointer, in which case a bad jump
could be generated. This problem is unlikely to happen, but it looks
possible.
JIT: minor comparison repairs

Fix some comparisions that are written as pointer comparisons
when they're actually integer comparisons. Also, remove an
unnecessarily slow variant of pointer compairson for x86_64.
JIT: better code for floating-point constants on x86+SSE