[jit] Fix state issue in gcse.c

[Steven Bosscher]

On Tue, Mar 11, 2014 at 8:00 PM, David Malcolm wrote:
> Investigation revealed the issue to be a CFG from the previous compile
> being kept alive by this GC root in gcse.c:
>           static GTY(()) rtx test_insn;
>
> This wouldn't it itself be an issue, but one (or more) of the edges had:

But this is an issue! This is not supposed to be possible.

test_insn is not in the insns chain and also not in a basic block, so
it should never keep a CFG alive.

Your patch papers over a bigger issue: How does test_insn end up
preventing the CFG from being garbage-collected.

Ciao!
Steven