ARM: crash in ARMAsmPrinter::GetARMGVSymbol

[google-yfyang]

Minimal reproducer:

target datalayout = "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
target triple = "armv7-unknown-linux-android23"

@llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @upb_GeneratedRegistry_Constructor_dont_copy_me__upb_internal_use_only, ptr null }]

; Uncommenting this block will resolve the crash.
;define void @test() {
;ret void
;}

declare void @upb_GeneratedRegistry_Constructor_dont_copy_me__upb_internal_use_only()

Instructions to reproduce:
llc < reduced.ll

The crash occurs on Subtarget field of ARMAsmPrinter inside of ARMAsmPrinter::GetARMGVSymbol. This is only initialized when runOnMachineFunction is called. That is why simply adding a function definition is "fixing" the crash.

[llvmbot]

@llvm/issue-subscribers-backend-arm

Author: None (google-yfyang)

Minimal reproducer: ``` target datalayout = "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64" target triple = "armv7-unknown-linux-android23"

@llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @upb_GeneratedRegistry_Constructor_dont_copy_me__upb_internal_use_only, ptr null }]

; Uncommenting this block will resolve the crash.
;define void @test() {
;ret void
;}

declare void @upb_GeneratedRegistry_Constructor_dont_copy_me__upb_internal_use_only()

Instructions to reproduce:
`llc <  reduced.ll`

The crash occurs on Subtarget field of ARMAsmPrinter inside of ARMAsmPrinter::GetARMGVSymbol. This is only initialized when runOnMachineFunction is called. That is why simply adding a function definition is "fixing" the crash. 

</details>

[davemgreen]

I believe there is a fix for this in #166329, as we saw it in #165422 too. I'll try and get that committed.