[AMDGPU] Allow MIMG with no uses in adjustWritemask in isel

Summary:
If an MIMG instruction has managed to get through to adjustWritemask in isel but
has no uses (and doesn't enable TFC) then prevent an assertion by not attempting
to adjust the writemask.

The instruction will be removed anyway.

Change-Id: I9a5dba6bafe1f35ac99c1b73df390936e2ac27a7

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58964

llvm-svn: 356540