AMDGPU: Refactor exp instructions

Structure the definitions a bit more like the other classes.

The main change here is to split EXP with the done bit set
to a separate opcode, so we can set mayLoad = 1 so that it won't
be reordered before the other exp stores, since this has the special
constraint that if the done bit is set then this should be the last
exp in she shader.

Previously all exp instructions were inferred to have unmodeled
side effects.

llvm-svn: 288695