Correct an assertion in LSRA.

Correct an assertion in LSRA.

`verifyFinalAllocation` asserts that if a non-BB interval RefPosition
that is either spilled or is the interval's last use does not have a
register, then that ref position must be marked `AllocateIfProfitable`.
However, this situation can also arise in at least one other situation:
an unused parameter will have at least one ref position that may not be
allocated to a register. This change corrects the assertion to check
`RefPosition::RequiresRegister` rather than
`RefPosition::AllocateIfProfitable`.

Fixes VSO 299207.