[PATCH] [PBQP] Teach PassConfig to tell if the default register allocator is used

[Arnaud A. de Grandmaison]

This enables targets to adapt their pass pipeline to the register allocator
in use.

 

For example, with the AArch64 backend, when using PBQP with cortex-a57, the
FPLoadBalancing pass is no longer necessary.

 

This patch addresses the last bits of the changes made by Lang Hames on
improving how targets can provide their own constraint builders @r219421,
and provides a generic way which can be used by other backends.

 

This is not as flexible as I would like it to be, as it can only tell if the
default allocator was superseded on the command line (we do not know which
one was actually selected), but this is simple enough for now and I doubt
many people are playing with more than 2 allocators at the same time.

 

Cheers,

--

Arnaud A. de Grandmaison

 
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