AMDGPU: Verify that SOP2/SOPC instructions have at most one immediate operand

Summary:
No test case because I don't know of a way to trigger this, but I
accidentally caused this to fail while working on a different change.

Change-Id: I8015aa447fe27163cc4e4902205a203bd44bf7e3

Reviewers: arsenm, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61490

llvm-svn: 360123