GlobalISel: Remove block insertion legalizer check

[arsenm]

Summary

This report was broken. The passed null block would crash in the
remark constructor.

I have a patch which will introduce a new block, and it seems to work
fine. It doesn't require legalizing any instructions in the new block,
so it's possible those will be missed.

[aemerson]

If we get a different number of blocks, can we instead check that the blocks have been legalized? If we get unlegalized instruction failures at selection time, it'll be confusing if the root cause is this.

[arsenm]

In D152397#4406862, @aemerson wrote:

If we get a different number of blocks, can we instead check that the blocks have been legalized? If we get unlegalized instruction failures at selection time, it'll be confusing if the root cause is this.

I basically see no value in trying to catch this with a special case fallback. First I don't actually know if the blocks will be missed, I'm just assuming. Anything attempted here is just going to be bug ridden and probably won't actually work, like this check didn't. I also don't think debugging that will be that difficult for whenever other block splitting legalizations are added

[bogner]

I think this is reasonable. If/when we hit a case where missing a more specific check here would be helpful to understand what went wrong we can add such a thing, but given that it's all hypothetical at this point and the current check is incorrect in some cases it makes sense to drop it.

[arsenm]

02d25c82b8b54a3c3f3791c700c7f4192554a95e

llvm/lib/CodeGen/GlobalISel/Legalizer.cpp

bool Legalizer::runOnMachineFunction(MachineFunction &MF) {
   MFResult Result = legalizeMachineFunction(MF, LI, AuxObservers, LocObserver,
                                             *MIRBuilder, KB);
 
   if (Result.FailedOn) {
     reportGISelFailure(MF, TPC, MORE, "gisel-legalize",
                        "unable to legalize instruction", *Result.FailedOn);
     return false;
   }
-  // For now don't support if new blocks are inserted - we would need to fix the
-  // outer loop for that.
-  if (MF.size() != NumBlocks) {
-    MachineOptimizationRemarkMissed R("gisel-legalize", "GISelFailure",
-                                      MF.getFunction().getSubprogram(),
-                                      /*MBB=*/nullptr);
-    R << "inserting blocks is not supported yet";
-    reportGISelFailure(MF, TPC, MORE, R);
-    return false;
-  }
 
   if (LocObserver.getNumLostDebugLocs()) {
     MachineOptimizationRemarkMissed R("gisel-legalize", "LostDebugLoc",
                                       MF.getFunction().getSubprogram(),
                                       /*MBB=*/&*MF.begin());
     R << "lost "
       << ore::NV("NumLostDebugLocs", LocObserver.getNumLostDebugLocs())
       << " debug locations during pass";