AMDGPU: Force skip over s_sendmsg and exp instructions

Summary:
These instructions interact with hardware blocks outside the shader core,
and they can have "scalar" side effects even when EXEC = 0. We don't
want these scalar side effects to occur when all lanes want to skip
these instructions, so always add the execz skip branch instruction
for basic blocks that contain them.

Also ensure that we skip scalar stores / atomics, though we don't
code-gen those yet.

Reviewers: arsenm, rampitec

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D48431

Change-Id: Ieaeb58352e2789ffd64745603c14970c60819d44
llvm-svn: 338235