Fix/work around PR57676, LRA terminates prematurely

[Jeff Law]

On 02/22/2016 07:34 AM, Richard Biener wrote:

> Hum, but then you get to "inifinite" compiles again when LRA is buggy
> or the user presents it with an impossible to handle asm.
Neither should be happening in practice, even an impossible asm should 
cause LRA to halt in some way or another.

In practice looping has occurred due to bugs in machine descriptions are 
are typically seen during development/porting.  Hence the desire to put 
it under -fchecking for gcc-6 and possibly implement something smarter 
for gcc-7 (where we'd track more precisely whether or not we're making 
forward progress).

>
> I don't think that's a good idea - maybe bumping the limit is the way to
> go instead?
No, because one just needs to build a longer chain of insns needing 
reloading.

>
> 30 constraint passes sounds excessive and a sign of a bug to me anyway.
Not really.  If you look at the testcase and the chain of reloads, it's 
legitimate.  Essentially each pass exposes a case where spill a register 
in an insn that previously had a register allocated.

Jeff