glsl2: Initialize ir_instruction::type and ir_rvalue::type.

glsl2: Initialize ir_instruction::type and ir_rvalue::type.

Top-level instructions now get NULL as their default type (since type is
irrelevant for things like ir_function), while ir_rvalues get error_type
by default.

This should make it easier to tell if we've forgotten to set a type.  It
also fixes some "Conditional jump or move depends on uninitialized
value" errors in valgrind caused by ir_validate examining the type of
top level ir_instructions, which weren't set.