See bug 40917: https://bugs.llvm.org/show_bug.cgi?id=40917
Shader programming documents state that MUBUF SOFFSET is "SGPR to supply unsigned byte offset. Must be an SGPR, M0 or inline constant."
Also note that our tests are based on SP3 tables which also allow inline constants.
Of course, using fp inline constants in this context is meaningless.