[AMDGPU] Fix 3 tests with expensive checks. NFC.

Image instructions are now not available for all targets anymore,
so a generic target cannot use it.

Author: rampitec

llvm/test/CodeGen/AMDGPU/sgpr-copy.ll

@@ -1,4 +1,4 @@
-; RUN:  llc -amdgpu-scalarize-global-loads=false  -march=amdgcn -verify-machineinstrs < %s | FileCheck %s
+; RUN:  llc -amdgpu-scalarize-global-loads=false  -march=amdgcn -mcpu=tahiti -verify-machineinstrs < %s | FileCheck %s
 ; RUN:  llc -amdgpu-scalarize-global-loads=false  -march=amdgcn -mcpu=tonga -mattr=-flat-for-global -verify-machineinstrs < %s | FileCheck %s
 
 ; CHECK-LABEL: {{^}}phi1:

llvm/test/CodeGen/AMDGPU/split-smrd.ll

@@ -1,4 +1,4 @@
-; RUN: llc -march=amdgcn -verify-machineinstrs < %s | FileCheck -check-prefix=GCN %s
+; RUN: llc -march=amdgcn -mcpu=tahiti -verify-machineinstrs < %s | FileCheck -check-prefix=GCN %s
 
 ; FIXME: Move this to sgpr-copy.ll when this is fixed on VI.
 ; Make sure that when we split an smrd instruction in order to move it to

llvm/test/CodeGen/AMDGPU/subreg-coalescer-crash.ll

@@ -1,4 +1,4 @@
-; RUN: llc -march=amdgcn -verify-machineinstrs -o - %s | FileCheck -check-prefix=GCN %s
+; RUN: llc -march=amdgcn -mcpu=tahiti -verify-machineinstrs -o - %s | FileCheck -check-prefix=GCN %s
 ; RUN: llc -march=amdgcn -mcpu=tonga -verify-machineinstrs -o - %s | FileCheck -check-prefix=GCN %s
 
 ; GCN-LABEL:{{^}}row_filter_C1_D0: