PR 32721, internal error in tc-i386.c:parse_register

pr30117 showed one of the assertions added by 4d1bb7955a8b was too
strict.  oss-fuzz also found the second assertion to be too strict,
with this testcase distilled from 7k of garbage source:

 A=%eax%%!
 Y=A
 Z=A
 or $6,Z

	PR 32721
	* config/tc-i386.c (parse_register): Move "know" into
	condition.  Simplify.