Factor out legalization of undef, and handle more cases for ARM.

tests_lit/llvm2ice_tests/nacl-atomic-intrinsics.ll

 ; This tests each of the supported NaCl atomic instructions for every
 ; size allowed.
 
 ; RUN: %p2i -i %s --filetype=obj --disassemble --args -O2 \
 ; RUN:   | FileCheck %s
 ; RUN: %p2i -i %s --filetype=obj --disassemble --args -O2 \
 ; RUN:   | FileCheck --check-prefix=O2 %s
 ; RUN: %p2i -i %s --filetype=obj --disassemble --args -Om1 \
 ; RUN:   | FileCheck %s
 
@@ skipping 753 matching lines @@
 ; CHECK: push ebx
 ; CHECK-DAG: mov edx
 ; CHECK-DAG: mov eax
 ; CHECK-DAG: mov ecx
 ; CHECK-DAG: mov ebx
 ; CHECK: lock cmpxchg8b QWORD PTR [e{{.[^x]}}+0x0]
 ; edx and eax are already the return registers, so they don't actually
 ; need to be reshuffled via movs. The next test stores the result
 ; somewhere, so in that case they do need to be mov'ed.
 
+define i64 @test_atomic_cmpxchg_64_undef(i32 %iptr, i64 %desired) {
+entry:
+  %ptr = inttoptr i32 %iptr to i64*
+  %old = call i64 @llvm.nacl.atomic.cmpxchg.i64(i64* %ptr, i64 undef,
+                                               i64 %desired, i32 6, i32 6)
+  ret i64 %old
+}
+; CHECK-LABEL: test_atomic_cmpxchg_64_undef
+; CHECK: lock cmpxchg8b QWORD PTR [e{{.[^x]}}+0x0]
+
 ; Test a case where %old really does need to be copied out of edx:eax.
 define void @test_atomic_cmpxchg_64_store(i32 %ret_iptr, i32 %iptr, i64 %expected, i64 %desired) {
 entry:
   %ptr = inttoptr i32 %iptr to i64*
   %old = call i64 @llvm.nacl.atomic.cmpxchg.i64(i64* %ptr, i64 %expected,
                                                 i64 %desired, i32 6, i32 6)
   %__6 = inttoptr i32 %ret_iptr to i64*
   store i64 %old, i64* %__6, align 1
   ret void
 }
@@ skipping 218 matching lines @@
   br i1 %cmp, label %done, label %body
 done:
   ret void
 }
 ; O2-LABEL: test_cmpxchg8b_regalloc
 ;;; eax and some other register will be used in the cmpxchg instruction.
 ; O2: lock cmpxchg8b QWORD PTR
 ;;; Make sure eax/ecx/edx/ebx aren't used again, e.g. as the induction variable.
 ; O2-NOT: {{eax|ecx|edx|ebx}}
 ; O2: pop ebx