[AMDGPU] Use PredicateControl in MIMGBaseOpcode. NFC.

This is infrastructural, will be needed for future work.
For some reason it was only used in MIMG_NoSampler, while
needed everywere we use MIMGBaseOpcode if we want to use
predicates.

Differential Revision: https://reviews.llvm.org/D66115

llvm-svn: 368626