Add option to force filetype=asm for testing

tests_lit/assembler/x86/jump_encodings.ll

 ; Tests various aspects of x86 branch encodings (near vs far,
 ; forward vs backward, using CFG labels, or local labels).
 
 ; Use -ffunction-sections so that the offsets reset for each function.
 ; RUN: %p2i --filetype=obj --disassemble -i %s --args -O2 \
 ; RUN:   -ffunction-sections | FileCheck %s
 
 ; Use atomic ops as filler, which shouldn't get optimized out.
 declare void @llvm.nacl.atomic.store.i32(i32, i32*, i32)
 declare i32 @llvm.nacl.atomic.load.i32(i32*, i32)
@@ skipping 122 matching lines @@
   %ptr = inttoptr i32 %iptr to i32*
   %cmp = icmp ult i32 %val, 1
   br i1 %cmp, label %next3, label %next2
 next2:
   call void @llvm.nacl.atomic.store.i32(i32 %val, i32* %ptr, i32 6)
   br label %next3
 next3:
   call void @llvm.nacl.atomic.store.i32(i32 %val, i32* %ptr, i32 6)
   br label %next1
 }
-; Forward branches for non-local labels currently use the fully relaxed
-; form to avoid needing a relaxation pass.
+; Note: forward branches for non-local labels in Subzero currently use the fully
+; relaxed form (4-byte offset) to avoid needing a relaxation pass.  When we use
+; llvm-mc, it performs the relaxation pass and uses a 1-byte offset.
 ; CHECK-LABEL: test_near_forward
-; CHECK:       8: {{.*}}            cmp
-; CHECK-NEXT:  b: 0f 82 05 00 00 00 jb 16
-; CHECK-NEXT: 11: {{.*}}            mov DWORD PTR
-; CHECK-NEXT: 13: {{.*}}            mfence
-; CHECK-NEXT: 16: {{.*}}            mov DWORD PTR
-; CHECK:      1b: eb eb             jmp 8
+; CHECK:      [[BACKLABEL:[0-9a-f]+]]: {{.*}} cmp
+; CHECK-NEXT: {{.*}} jb [[FORWARDLABEL:[0-9a-f]+]]
+; CHECK-NEXT: {{.*}} mov DWORD PTR
+; CHECK-NEXT: {{.*}} mfence
+; CHECK-NEXT: [[FORWARDLABEL]]: {{.*}} mov DWORD PTR
+; CHECK:      {{.*}} jmp [[BACKLABEL]]
 
 
 ; Unlike forward branches to cfg nodes, "local" forward branches
 ; always use a 1 byte displacement.
 ; Check local forward branches, followed by a near backward branch
 ; to make sure that the instruction size accounting for the forward
 ; branches are correct, by the time the backward branch is hit.
 ; A 64-bit compare happens to use local forward branches.
 define internal void @test_local_forward_then_back(i64 %val64, i32 %iptr,
                                                    i32 %val) {
@@ skipping 31 matching lines @@
   %success = icmp eq i32 1, %a
   br i1 %success, label %next, label %next2
 }
 ; CHECK-LABEL: test_local_backward
 ; CHECK:       9: {{.*}} mov {{.*}},DWORD
 ; CHECK:       b: {{.*}} mov
 ; CHECK-NEXT:  d: {{.*}} xor
 ; CHECK-NEXT:  f: {{.*}} lock cmpxchg
 ; CHECK-NEXT: 13: 75 f6 jne b
 ; CHECK:      1c: 74 eb je 9