AMDGPU/SI: Change predicate to isCIOnly for 32-bit imm s_buffer_load* patterns

Summary:
This is essentially NFC, because the complex pattern used for these patterns
will fail on non-CI, but this makes the pattern consistent with other CI
smrd patterns.  It is also a performance improvement, because the pattern
will now fail earlier on non-CI.

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D52469

llvm-svn: 343125