[AMDGPU, test] Fix use of undef FileCheck var

Test CodeGen/AMDGPU/amdgpu.private-memory.ll and
CodeGen/AMDGPU/private-memory-r600.ll have a block of CHECK directives
whose prefix is inconsistent: R600-CHECK Vs R600. This leads to a
R600-NOT directive using an undefined CHAN variable due to R600-CHECK
directives never being considered by FileCheck. Fixing the prefix leads
to the testcase failing. As per https://reviews.llvm.org/D99865#2675235
this commit removes the directives instead since it is not possible to
write a reliable check.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D99865