implement the null function for the Mips32 subzero compiler

tests_lit/llvm2ice_tests/function_aligned.ll

Index: tests_lit/llvm2ice_tests/function_aligned.ll
diff --git a/tests_lit/llvm2ice_tests/function_aligned.ll b/tests_lit/llvm2ice_tests/function_aligned.ll
index 23d2ca75ea465335aa5929f6cb9f83a8a1a14caf..763e919f252bc7e7aeefcf6f6f74d71d0322eba2 100644
--- a/tests_lit/llvm2ice_tests/function_aligned.ll
+++ b/tests_lit/llvm2ice_tests/function_aligned.ll
@@ -13,6 +13,9 @@
 ; RUN:   --disassemble --target arm32 -i %s --args -O2 --skip-unimplemented \
 ; RUN:   | %if --need=target_ARM32 --need=allow_dump \
 ; RUN:   --command FileCheck --check-prefix ARM32 %s
+; RUN: %if --need=target_MIPS32 --need=allow_dump --command %p2i --filetype=asm --assemble \

[jvoung (off chromium), 2015/07/08 00:39:12]

Since there is "\" at the end of the line to wrap and avoid spilling over 80
cols, you could wrap earlier. Otherwise this is now spilling over 80 cols.

E.g., could wrap earlier like the ARM versions.

+; RUN:   --disassemble --target mips32 -i %s --args -O2 --skip-unimplemented \
+; RUN:   | %if --need=target_MIPS32 --need=allow_dump --command FileCheck --check-prefix MIPS32 %s

[jvoung (off chromium), 2015/07/08 00:39:12]

similar, could wrap earlier

 
 define void @foo() {
   ret void
@@ -25,6 +28,12 @@ define void @foo() {
 ; ARM32-NEXT: 4: e7fedef0 udf
 ; ARM32-NEXT: 8: e7fedef0 udf
 ; ARM32-NEXT: c: e7fedef0 udf
+; MIPS32-LABEL: foo
+; MIPS32: 4: {{.*}} jr ra
+; MIPS32-NEXT: 8: {{.*}} nop
+; MIPS32-NEXT: c: e7fedef0
+; TODO (reed kotler check this disassembly, .s file looks fine)

[jvoung (off chromium), 2015/07/08 00:39:12]

"TODO (reed kotler" -> "TODO(reed kotler)"

+; "e7fedef0" is an "undefined"/halting instruction for ARM (what llvm.trap would translate to)
 
 define void @bar() {
   ret void
@@ -33,3 +42,7 @@ define void @bar() {
 ; CHECK-NEXT: 20: {{.*}} ret
 ; ARM32-LABEL: bar
 ; ARM32-NEXT: 10: {{.*}} bx lr
+; TOOD (Reed Kotler check this disassembly, .s file looks fine)
+; MIPS32-LABEL: bar
+; MIPS32: 14: {{.*}} jr ra
+; MIPS32-NEXT: 18: {{.*}} nop