feat(ubtb): remove takenCnt & valid field

[ngc7331]

We only train taken branches into ubtb, if it's not taken, it's not useful. So usefulCnt is some sort of takenCnt. Therefore, we can remove takenCnt and do an always-taken prediction in ubtb.

This also allows non-taken branches to be evicted earlier and thus we can use abtb/fallthrough results, so maybe has some performance gain.

We can also remove valid as it should always be usefulCnt != 0

[ngc7331]

CI IPC has increased both with/without fastTrain (#5145)

IPC report w/o #5145

Run #	Branch	Commit	PR	Updated	astar	copy_and_run	coremark	gcc	gromacs	hmmer-Vector	lbm	linux	mcf	microbench	milc	namd	povray	wrf	xalancbmk	Avg. Diff
18934432033 (base)	kunminghu-v3	82957fb	unknown	2025-10-31T10:48:11Z	0.762649	0.439234	1.004732	0.65648	3.003717	1.62862	2.529884	1.110032	0.713159	0.731378	1.419134	2.182647	1.180872	1.894855	0.839904
18965831697	feat-ubtb-remove-takencnt	56ae437	#5157	2025-11-01T06:31:30Z	0.777731	0.438955	1.007372	0.658527	3.002407	1.626572	2.529884	1.104146	0.731181	0.727204	1.463042	2.439043	1.177436	1.897886	0.879549
18965831697 diff					1.98%	-0.06%	0.26%	0.31%	-0.04%	-0.13%	0.00%	-0.53%	2.53%	-0.57%	3.09%	11.75%	-0.29%	0.16%	4.72%	1.55%

IPC report w/ #5145

Run #	Branch	Commit	PR	Updated	astar	copy_and_run	coremark	gcc	gromacs	hmmer-Vector	lbm	linux	mcf	microbench	milc	namd	povray	wrf	xalancbmk	Avg. Diff
18871592359 (base)	feat-ubtb-use-fasttrain	14f4a29	#5145	2025-10-28T19:39:31Z	0.807507	0.464202	1.139789	0.730261	2.997342	1.735806	2.535753	1.164367	0.695311	0.847041	1.675045	3.354448	1.353185	2.007131	0.970846
18935641794	feat-ubtb-remove-takencnt	86dddb7	#5157	2025-10-30T21:12:27Z	0.827856	0.465855	1.120577	0.731361	3.006895	1.740222	2.532403	1.166595	0.760284	0.850924	1.675045	3.378833	1.333953	1.995335	0.970255
18935641794 diff					2.52%	0.36%	-1.69%	0.15%	0.32%	0.25%	-0.13%	0.19%	9.34%	0.46%	0.00%	0.73%	-1.42%	-0.59%	-0.06%	0.70%

[XiangShanRobot]

[Generated by IPC robot]
commit: 56ae437

commit	astar	astar_simfrontend	copy_and_run	coremark	gcc	gromacs	hmmer-Vector	lbm	linux	mcf	microbench	milc	milc_simfrontend	namd	povray	wrf	xalancbmk	xalancbmk_simfrontend
56ae437

kunminghu-v3 branch:

commit	astar	astar_simfrontend	copy_and_run	coremark	gcc	gromacs	hmmer-Vector	lbm	linux	mcf	microbench	milc	milc_simfrontend	namd	povray	wrf	xalancbmk	xalancbmk_simfrontend
e088701
526430a
82957fb
6a53198
123d301
2e4c95e
ea82e26	0.763	2.138	0.439	1.005	0.659	3.004	1.629	2.530	1.110	0.713	0.731	1.419	1.638	2.183	1.181	1.895	0.840	3.448
44eaf57	0.763	2.138	0.439	1.005	0.659	3.004	1.629	2.530	1.110	0.713	0.731	1.419	1.638	2.183	1.181	1.895	0.840	3.448
c48286e	0.763	2.138	0.439	1.005	0.658	3.004	1.629	2.530	1.110	0.713	0.731	1.419	1.638	2.183	1.181	1.895	0.840	3.448
ba3c5f2	0.763	2.138	0.439	1.005	0.661	3.004	1.629	2.530	1.110	0.713	0.731	1.419	1.638	2.183	1.181	1.895	0.840	3.448