[AMDGPU] Derive GCNSubtarget from MF to get overridden target features

Summary:
AMDGPUAsmPrinter has a getSTI function that derives a GCNSubtarget from the
TM. However, this means that overridden target features are not detected and can
result in incorrect behaviour.

Switch to using STM which is a GCNSubtarget derived from the MF (used elsewhere
in the same function).

Change-Id: Ib6328ad667b7fcdc87e9c06344e59859207db9b0

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

Differential Revision: https://reviews.llvm.org/D54301

llvm-svn: 347221