radeon/llvm: Use multiclasses for floating point loads

radeon/llvm: Use multiclasses for floating point loads

The original strategy for handling floating point loads, which was to
lower (f32 load) to (f32 bitcast (i32 load)) wasn't really working.  The
main problem was that the DAG legalizer couldn't handle replacing a node
with two results (load) with a node with only one result (bitcast).