AMDGPU: Avoid folding 2 constant operands into an SALU operation

Summary:
Catch the (admittedly unusual) case where SIFoldOperands attempts to fold 2
constant operands into the same SALU operation, with neither operand able to be
encoded as an inline constant.

Change-Id: Ibc48d662c9ffd8bbacd154976b0b1c257ace0927

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70896