AMDGPU: Use PatFrags to allow selecting custom nodes or intrinsics

This enables GlobalISel to handle various intrinsics. The custom node
pattern will be ignored, and the intrinsic will work. This will also
allow SelectionDAG to directly select the intrinsics, but as they are
all custom lowered to the nodes, this ends up leaving dead code in the
table.

Eventually either GlobalISel should add the equivalent of custom nodes
equivalent, or intrinsics should be directly used. These each have
different tradeoffs.

There are a few more to handle, but these are easy to handle
ones. Some others fail for other reasons.

llvm-svn: 371432