[BreakFalseDeps][X86] Move operand loop out of X86's getUndefRegClearance and put in the pass.

X86 is the only user of this interface in tree. Previously the
X86 pass would loop over operands looking for one undef operand for
the pass to fix. But there could theoretically be multiple operands
to fix. So it makes more sense for the pass to do the looping and
ask the target if an operand needs to be fixed.