Revert "Reapply r291025 ("AMDGPU: Remove unneccessary intermediate vector")"

Summary: This reverts commit r291144. It breaks build bots.

http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-autoconf/builds/3270, http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fuzzer/builds/2058

lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp:1638:12: error: could not convert ‘(const unsigned int*)(& Variants)’ from ‘const unsigned int*’ to ‘llvm::ArrayRef<unsigned int>’
     return Variants;

Reviewers: eugenis, tstellarAMD

Patch by Alex Shlyapnikov.

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, tony-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D28372

llvm-svn: 291168