AMDGPU/SI: When splitting SMRD instructions, add its users to VALU worklist

Summary:
When we split SMRD instructions into two MUBUFs we were adding the users
of the newly created MUBUFs to the VALU worklist.  However, the only
users these instructions had was the REG_SEQUENCE that was inserted
by splitSMRD when the original SMRD instruction was split.

We need to make sure to add the users of the original SMRD to the VALU
worklist before it is split.

I have a test case, but it requires one other bug fix, so it will be
added in a later commt.

Reviewers: mareko, arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D17101

llvm-svn: 260588