x86/gas: replace inappropriate assertion when parsing registers

x86/gas: replace inappropriate assertion when parsing registers

PR gas/30117
Once a symbol had its expression evaluated, the "segment" of the symbol
may be reg_section if a register is merely involved in the expression,
not just when the expression references a "plain" register. Therefore
the first of the assertions put in place by 4d1bb7955a8b was too strict.
Convert it to an if() to deal with situations like this one found by
fuzzing:

	x=s
	s=%eax+0
	y=s
	or $6,x

In non-debug builds this also avoids potentially silently generating bad
code.