[PPCGCodeGeneration] Fix runtime check adjustments since they make assumptions about BB layout.

- There is a conditional branch that is used to switch between the old
   and new versions of the code.

- If we detect that the build was unsuccessful, `PPCGCodeGeneration` will
  change the runtime check to be always set to false.

- To actually *reach* this runtime check instruction, `PPCGCodeGeneration`
  was using assumptions about the layout of the BBs.

- However, invariant load hoisting violates this assumption by inserting
  an extra basic block in the middle.

- Fix the assumption on the layout by having `createScopConditionally`
   return the conditional branch instruction.

- Use this reference to set to always-false.

llvm-svn: 308010