[SPARC] Implement hooks for conditional branch relaxation

Integrate the BranchRelaxation pass to help with relaxing out-of-range
conditional branches.

This is mostly of concern for SPARCv9, which uses conditional branches with
much smaller range than its v8 counterparts.
(Some large autogenerated code, such as the ones generated by TableGen, already
hits this limitation when building in Release)

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D142458