[PR50869] don't attempt to expand CFA within cselib

[Alexandre Oliva]

An assertion check meant to verify that var loc expansions that didn't
involve VALUEs (say constants, REGs, etc) didn't push values onto the
dependency stack failed in an expansion of the argp reg, because
equivalences for it are preserved at cselib table resets, and cselib
later tries to expand it to equivalent expressions.

It's not profitable to expand it within var-tracking, and that's the
only user of the CFA-base special-casing in cselib, so I arranged for
argp to be preserved in expansions, just like other stack base
registers.

While debugging it, I noticed it was theoretically possible for the
expression depth to remain uninitialized, and added an initialization
and an assertion check to make sure it only remains zero when no
location is found.

Regstrapped on x86_64-linux-gnu and i686-linux-gnu.  Ok to install?

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-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer