[NFC][SPIRV] Make the zero-length-array.ll test explicit about what is generated (#166910)

This patch doesn't change anything. Just adds more explicit checks to
verify what is generated in this case when an alloca has a zero-sized
array.

I'd expect an `OpRuntimeArray`, but nothing is generated.

Author: jmmartinez

llvm/test/CodeGen/SPIRV/zero-length-array.ll

@@ -1,10 +1,17 @@
 ; RUN: llc -verify-machineinstrs -O0 -mtriple=spirv-unknown-vulkan-compute %s -o - | FileCheck %s
 ; RUN: %if spirv-tools %{ llc -O0 -mtriple=spirv-unknown-vulkan-compute %s -o - -filetype=obj | spirv-val %}
 
-; CHECK: %[[#type:]] = OpTypeInt 32 0
-; CHECK: %[[#ext:]] = OpConstant %[[#type]] 0
+; Nothing is generated, but compilation doesn't crash.
+; CHECK: OpName %[[#FOO:]] "foo"
+; CHECK: OpName %[[#RTM:]] "reg2mem alloca point"
+; CHECK: %[[#INT:]] = OpTypeInt 32 0
+; CHECK: %[[#RTM]] = OpConstant %[[#INT]] 0
+; CHECK: %[[#FOO]] = OpFunction
+; CHECK-NEXT: = OpLabel
+; CHECK-NEXT: OpReturn
+; CHECK-NEXT: OpFunctionEnd
 
-define spir_func void @_Z3foov() {
+define spir_func void @foo() {
 entry:
   %i = alloca [0 x i32], align 4
   ret void