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

[jobnoorman]

Summary

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.

[StephenFan]

LGTM.

llvm/lib/ExecutionEngine/JITLink/ELF_riscv.cpp

if (I < Aux.RelaxEdges.size() && Aux.RelaxEdges[I] == &E) {
       Delta = Aux.RelocDeltas[I];
       ++I;
     }
   }
 
   // Remove AlignRelaxable edges: all other relaxable edges got modified and
   // will be used later while linking. Alignment is entirely handled here so we
   // don't need these edges anymore.
-  for (auto *B : G.blocks()) {
-    for (auto IE = B->edges().begin(); IE != B->edges().end();) {
-      if (IE->getKind() == AlignRelaxable)
-        IE = B->removeEdge(IE);
-      else
-        ++IE;
-    }
-  }
-}
+  for (auto IE = Block.edges().begin(); IE != Block.edges().end();) {
+      IE = Block.removeEdge(IE);
 
 static void finalizeRelax(LinkGraph &G, RelaxAux &Aux) {
   for (auto &[B, BlockAux] : Aux.Blocks)
     finalizeBlockRelax(G, *B, BlockAux);
 }
 
 static Error relax(LinkGraph &G) {
   auto Aux = initRelaxAux(G);