[llvm] [BOLT] Add getter for optional relocations (PR #133085)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

Adds the `isOptional` getter, which I forgot to add in #131638. User will be:
- #116964

(cc: @maksfb)

https://github.com/llvm/llvm-project/pull/133085