[LocalStackSlotAllocation] Minor simplifications. NFC

Instead of maintaining a separate valid flag for BaseReg, Use
BaseReg.isValid(). I think this is left over from an older
implementation that maintained a vector of base registers.

The other change is not do a speculative assignment to BaseOffset
that needs to be reverted. Only commit it after we do the check.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D141153