Cranelift: Simplify all our various CLIF load and store instructions

[fitzgen]

• {u,s}load{8,16,32} these are all equivalent to load.i{8,16,32} followed by {u,s}extend. We should just use that larger sequence and backends that have special lowerings for uload8() that are better than uextend(load8()) should add rules to match the compound pattern.

• Ditto for {u,s}store{8,16,32}.

• Same for our SIMD loads like uload16x4 et al. These can be load.i16x4 followed by a lane-wise uextend, etc.