[AArch64][test] Fix test failing on unknown options (llvm#139696)

Fixes buildbot failure
https://lab.llvm.org/buildbot/#/builders/16/builds/18873 originating
from llvm#138448. Normally ignored silently but fails on higher error
levels.

Buildbot errors:
```
/b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/llc < /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AArch64/reserveXreg.ll -mtriple=aarch64-unknown-linux-gnu | /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/FileCheck /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AArch64/reserveXreg.ll # RUN: at line 6
+ /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/FileCheck /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AArch64/reserveXreg.ll
+ /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/llc -mtriple=aarch64-unknown-linux-gnu
'+reserve-x8' is not a recognized feature for this target (ignoring feature)
'+reserve-x8' is not a recognized feature for this target (ignoring feature)
'+reserve-x16' is not a recognized feature for this target (ignoring feature)
'+reserve-x16' is not a recognized feature for this target (ignoring feature)
'+reserve-x17' is not a recognized feature for this target (ignoring feature)
'+reserve-x17' is not a recognized feature for this target (ignoring feature)
```

Author: Nuko Y.

llvm/test/CodeGen/AArch64/reserveXreg.ll

@@ -1,7 +1,6 @@
 ;; Check if manually reserved registers are always excluded from being saved by
 ;; the function prolog/epilog, even for callee-saved ones, as per GCC behavior.
-;; X19(BP, LLVM specific), X29(FP), X30(LP), X31(SP) are special so
-;; they are not checked.
+;; Look at AArch64Features.td for registers excluded from this test.
 
 ; RUN: llc < %s -mtriple=aarch64-unknown-linux-gnu | FileCheck %s
 
@@ -82,17 +81,6 @@ define preserve_mostcc void @t7() "target-features"="+reserve-x7" {
   ret void
 }
 
-define preserve_mostcc void @t8() "target-features"="+reserve-x8" {
-; CHECK-LABEL: t8:
-; CHECK: // %bb.0:
-; CHECK-NEXT:        mov     w8, #256
-; CHECK-NEXT:        //APP
-; CHECK-NEXT:        //NO_APP
-; CHECK-NEXT:        ret
-  call i64 asm sideeffect "", "={x8},{x8}"(i64 256)
-  ret void
-}
-
 define preserve_mostcc void @t9() "target-features"="+reserve-x9" {
 ; CHECK-LABEL: t9:
 ; CHECK: // %bb.0:
@@ -170,28 +158,6 @@ define preserve_mostcc void @t15() "target-features"="+reserve-x15" {
   ret void
 }
 
-define preserve_mostcc void @t16() "target-features"="+reserve-x16" {
-; CHECK-LABEL: t16:
-; CHECK: // %bb.0:
-; CHECK-NEXT:        mov     w16, #256
-; CHECK-NEXT:        //APP
-; CHECK-NEXT:        //NO_APP
-; CHECK-NEXT:        ret
-  call i64 asm sideeffect "", "={x16},{x16}"(i64 256)
-  ret void
-}
-
-define preserve_mostcc void @t17() "target-features"="+reserve-x17" {
-; CHECK-LABEL: t17:
-; CHECK: // %bb.0:
-; CHECK-NEXT:        mov     w17, #256
-; CHECK-NEXT:        //APP
-; CHECK-NEXT:        //NO_APP
-; CHECK-NEXT:        ret
-  call i64 asm sideeffect "", "={x17},{x17}"(i64 256)
-  ret void
-}
-
 define preserve_mostcc void @t18() "target-features"="+reserve-x18" {
 ; CHECK-LABEL: t18:
 ; CHECK: // %bb.0: