[JITLink][RISCV] Fix use-after-free in relax

Finalization of relaxation calls `finalizeBlockRelax` for every block in
the graph. This function, however, would iterate over //all// blocks in
the graph to remove `AlignRelaxable` edges. Since pointers to those
edges would still be stored in `RelaxEdges`, this caused a
use-after-free for graphs with multiple blocks.

This patch fixes this by only iterating over the edges of the current
block in `finalizeBlockRelax`.

Reviewed By: StephenFan

Differential Revision: https://reviews.llvm.org/D154844