[FastISel] Fix the crash in gc.result lowering

[skatkov]

Summary

The Fast ISel has a fallback to SelectionDAGISel in case it cannot handle the instruction.
This works as follows:
Using reverse order, try to select instruction using Fast ISel, if it cannot handle instruction it fallbacks to SelectionDAGISel
for these instructions if it is a call and continue fast instruction selections.

However if unhandled instruction is not a call or statepoint related instruction it fallbacks to SelectionDAGISel for all remaining
instructions in basic block.

However gc.result instruction is missed and as a result it is possible that gc.result is processed earlier than statepoint
causing breakage invariant the gc.results should be handled after statepoint.

Test is updated because in the current form fast-isel cannot handle ret instruction (due to i1 ret type without explicit ext)
and as a result test does not check isel at all.

[reames]

LGTM

lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

if (FastIS) {
 
         FastISelFailed = true;
 
         // Then handle certain instructions as single-LLVM-Instruction blocks.
         // We cannot separate out GCrelocates to their own blocks since we need
         // to keep track of gc-relocates for a particular gc-statepoint. This is
         // done by SelectionDAGBuilder::LowerAsSTATEPOINT, called before
         // visitGCRelocate.
-        if (isa<CallInst>(Inst) && !isStatepoint(Inst) && !isGCRelocate(Inst)) {
+        if (isa<CallInst>(Inst) && !isStatepoint(Inst) && !isGCRelocate(Inst) &&
+            !isGCResult(Inst)) {
           OptimizationRemarkMissed R("sdagisel", "FastISelFailure",
                                      Inst->getDebugLoc(), LLVMBB);
 
           R << "FastISel missed call";
 
           if (R.isEnabled() || EnableFastISelAbort) {
             std::string InstStrStorage;
             raw_string_ostream InstStr(InstStrStorage);

test/CodeGen/X86/fast-isel-gc-intrinsics.ll

entry:
   ret i8 addrspace(1)* %vnew
 }
 
 ; gcresult calls are fine in their own blocks.
 define i1 @test_gcresult() gc "statepoint-example" {
 entry:
   %safepoint_token = tail call token (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 0, i1 ()* @return_i1, i32 0, i32 0, i32 0, i32 0)
   %call1 = call zeroext i1 @llvm.experimental.gc.result.i1(token %safepoint_token)
+  br label %exit
+exit:
   ret i1 %call1
 }
 
 ; we are okay here because we see the gcrelocate and avoid generating their own
 ; block.
 define i1 @test_gcresult_gcrelocate(i8 addrspace(1)* %v) gc "statepoint-example" {
 entry:
   %safepoint_token = tail call token (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 0, i1 ()* @return_i1, i32 0, i32 0, i32 0, i32 0, i8 addrspace(1)* %v)