[LoopUnroll] Make UnrollMaxUpperBound to be overridable by target (#76029)

The UnrollMaxUpperBound should be target dependent, since different
chips provide different register set which brings different ability of
storing more temporary values of a program. So I add a MaxUpperBound
value in UnrollingPreference which can be override by targets. All uses
of UnrollMaxUpperBound are replaced with UP.MaxUpperBound.
 
The default value is still 8 and the command line argument
'--unroll-max-upperbound' takes final effect if provided.