enable checking for stage1 compiler

[Nathan Sidwell]

Mark Mitchell wrote:

> I agree.
> This patch probably will not help us find bugs we would not otherwise 
> find, since we already have checking on by default on the mainline.  It 

I believe it will help find bugs, because on a --disable-checking build,
the checking stage1 compiler will be compiling a different program than
the stage1 compiler of an --enable-checking build compiles. -- All the
asserts will be removed, giving opportunities for other optimizations,
possibly.  The current --disable-checking bootstrap failure is the
exposure of just such a bug (unfortunately it is not caught by the self
checks :(.

I think this is important for a --disable-checking bootstrap.  How
about if only ENABLE_ASSERT_CHECKING was forced on at stage1?

> However, I think that the SSA change is probably worthwhile anyhow, as 
> the kinds of problems that Daniel refers to are indeed serious; they 
> will make it much harder to reproduce problems in releases with 
> --enable-checking compilers.

ok, I'll commit that bit.

nathan
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