rfc: clobber all call-saved registers

[Richard Henderson]

On two instances in the last week, I've created reduced test
cases in which it was necessary to convince reload that it
could not put a particular value into a hard register in order
to elicit the problem.  In both instances I had to create a
platform-specific test that used an asm to clobber all the
registers in question.

I hate creating platform specific tests for potentially generic
problems, since the problem is bound to show up somewhere else
as well.  So what to do?  I can think of two options:

(1) A header file somewhere in the testsuite that has a whole
    whale-load of ifdefs and chooses the correct asm statement,

(2) A compiler builtin that generates the correct asm statement
    based on the contents of call_used[].

Thoughts?


r~