[BreakFalseDeps] Harden pickBestRegisterForUndef against changing tied...

[BreakFalseDeps] Harden pickBestRegisterForUndef against changing tied operands or physical registers that aren't renamable.

I don't have any test cases since X86 doesn't return any tied
operands from getUndefRegClearance today. But conceivably we could
want BreakFalseDeps to insert a dependency breaking XOR for
a tied operand in the future.