[AArch64] Adding "armv8.8-a" BC instruction.

This instruction is described in the Arm A64 Instruction Set
Architecture documentation available here:
https://developer.arm.com/documentation/ddi0596/2021-12/Base-Instructions/BC-cond--Branch-Consistent-conditionally-?lang=en

FEAT_HBC "Hinted Conditional Branches" is listed in the 2021 A-Profile Architecture Extensions:
https://developer.arm.com/architectures/cpu-architecture/a-profile/exploration-tools/feature-names-for-a-profile

'BC.cc', where 'cc' is any ordinary condition code, is an instruction
that looks exactly like B.cc (the normal conditional branch), except
that bit 4 of the encoding is 1 rather than 0, which hints something
to the branch predictor (specifically, that this branch is expected to
be highly consistent, even though _which way_ it will consistently go
is not known at compile time).

This commit introduces a special subtarget feature for HBC, which is a
dependency of the top-level 8.8-A feature, and uses that to enable the
new BC instruction.

Differential Revision: https://reviews.llvm.org/D116156