glsl: Let ir_builder expressions take un-dereferenced variables.

glsl: Let ir_builder expressions take un-dereferenced variables.

Having to explicitly dereference is irritating and bloats the code,
when the compiler can detect and do the right thing.

v2: Use a little shim class to produce the automatic dereference
    generation at compile time as opposed to runtime, while also
    allowing compile-time type checking.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>