[llvm] [BOLT][AArch64] Support FEAT_CMPBR branch instructions. (PR #174972)

[Maksim Panchenko via llvm-commits]

maksfb wrote:

> @maksfb something similar to what we discussed in BOLT office hours (relax compare-and-branch into compare followed by branch) was done in the compiler with #116465. 

In BOLT, we will have to analyze for the conditional flag liveness in order to convert into `cmp`+`b.cc`.  Or relax using an extra `b`, like other short branches.


https://github.com/llvm/llvm-project/pull/174972