Fold the partition select into the sized-free tag compare.

[copybara-service]

Fold the partition select into the sized-free tag compare.

do_free_with_size previously tested (uptr & kNormalOrBadDeallocationMask) !=
kNormalMask (accepting both kNormalP0 and kNormalP1 via their shared bit) and
then had fast_free_with_size re-derive the partition from the pointer with
PartitionFromPointerFast (x86: btq $43; jb / aarch64: tbnz).

Heap partitioning defaults to off, so essentially every sized free is P0.
Widen the mask to kTagOrBadDeallocationMask and compare against the exact
kNormalP0 tag value: the not-taken fallthrough now is P0, and the partition is
known without a second test. On the taken side, test the exact kNormalP1 value
first (a partitioned heap still pays two compares, as before), then nullptr
(tag 0 fails both compares), cold, and the sampled/illformed handler.
fast_free_with_size takes the partition as a template parameter, which also
removes the duplicated GetSizeClass/FreeSmall arms (b/470136917).

The fold is sound because the tag field of kTagOrBadDeallocationMask is
disjoint from the high-bits and alignment fields and the exact kNormalP0 /
kNormalP1 values lie entirely within kTagMask; static_asserts check both, plus
that neither tag value is 0 so nullptr is rejected. kNormalPartitions == 1
builds compile the P1 test out. A new test covers nullptr through every sized
deallocation entry point.

x86 TCMallocInternalDeleteSized drops btq/jb (-2 instructions, -1 cmp/-1 jcc in
the fast_path goldens for delete(size) and delete(size,align)); aarch64 drops
the tbnz (-1 jcc). The cold-hint path gains one compare (cycles unchanged in
the benchmark).

The only other golden delta is new(cold,token_1)->size in the two
release_base_opt goldens (jcc -1, cmov +1). That row already differs from the
checked-in golden when built from unmodified HEAD sources (FDO drift), so it is
picked up by the regeneration but is not caused by this change.