AMDGPU: Insert two S_NOP instructions for every high level source statement.

Patch by: Konstantin Zhuravlyov

Summary: Tools, such as debugger, need to pause execution based on user input (i.e. breakpoint). In order to do this, two S_NOP instructions are inserted for each high level source statement: one before first isa instruction of high level source statement, and one after last isa instruction of high level source statement. Further, debugger may replace S_NOP instructions with S_TRAP instructions based on user input.

Reviewers: tstellarAMD, arsenm

Subscribers: echristo, dblaikie, arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D17454

llvm-svn: 262579