[RISCV][test] Update branch-relaxation.ll with update_llc_test_checks.py [NFC]

[RISCV][test] Update branch-relaxation.ll with update_llc_test_checks.py [NFC]

Update `llvm/test/CodeGen/RISCV/branch-relaxation.ll` with
`update_llc_test_checks.py`, according to
https://reviews.llvm.org/D130560#3746417:

>>! In D130560#3746417, @luismarques wrote:
>>>! In D130560#3746379, @luismarques wrote:
>> The tests don't seem to have been properly updated with
>> `update_llc_test_checks.py`.
>> `llvm/test/CodeGen/RISCV/branch-relaxation.ll` contains RV64 RUN
>> lines but the corresponding CHECK lines are missing in
>> some functions.
>
> Looking more closely at this, I guess you tried to only include the
> `CHECK-RV64` and `CHECK-RV32` checks when relevant. That's a good
> instinct but I guess it goes a bit against how we normally use
> `update_llc_test_checks.py`. My understanding of the trade-off of
> using that tool is that the test updates are much easier, even if
> sometimes the CHECKs aren't as tight as something more tailormade.