[AMDGPU] Add perf hints to functions

This is adoption of HSAIL perfhint pass. Two types of hints are produced:

1. Function is memory bound.
2. Kernel can use wave limiter.

Currently these hints are used in the scheduler. If a function is suspected
to be memory bound we allow occupancy to decrease to 4 waves in the course
of scheduling.

Differential Revision: https://reviews.llvm.org/D46992

llvm-svn: 333289