R600/SI: Make sure non-inline constants aren't folded into mubuf soffset operand

R600/SI: Make sure non-inline constants aren't folded into mubuf soffset operand

mubuf instructions now define the soffset field using the SCSrc_32
register class which indicates that only SGPRs and inline constants
are allowed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224622 91177308-0d34-0410-b5e6-96231b3b80d8