AMDGPU/SI: Implement GroupStaticSize Intrinsic for Dynamic LDS

Summary:
  Static LDS size is saved in MachineFunctionInfo::LDSSize,
We define a pseudo instruction with usesCustomInserter bit set. Then, in EmitInstrWithCustomInserter,
we replace this pseudo instruction with a mov of MachineFunctionInfo::LDSSize.

Reviewers:
    arsenm
    tstellarAMD

Subscribers
    llvm-commits, arsenm

Differential Revision:
   http://reviews.llvm.org/D18064

llvm-svn: 263563