[patch] combine ICE fix

[Cesar Philippidis <cesar at codesourcery dot com>]

This patch addresses an ICE when building qemu for a Mips target in
Yocto. Both gcc-trunk, gcc-4.8 and all of the targets are potentially
affected. The problem occurs because the instruction combine phase uses
two data structures to keep track of registers, reg_stat and
regstat_n_sets_and_refs, but they potentially end up out of sync; when
combine inserts a new register into reg_stat it does not update
regstat_n_sets_and_refs. Failing to update the latter results in an
occasional segmentation fault.

Is this OK for trunk and gcc-4.8? If so, please check it in. I tested it
on Mips and x86-64 and no regressions showed up.

Thanks,
Cesar

Attachment: gcc_combine_qemu_mips.diff
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