glsl: Create an ir_builder helper for hand-generating IR.

glsl: Create an ir_builder helper for hand-generating IR.

The C++ constructors with placement new, while functional, are
extremely verbose, leading to generation of simple GLSL IR expressions
like (a * b + c * d) expanding to many lines of code and using lots of
temporary variables.  By creating a new ir_builder.h that puts simple
generators in our namespace and taking advantage of ralloc_parent(),
we can generate much more compact code, at a minor runtime cost.

v2: Replace ir_instruction usage with just ir_rvalue.
v3: Drop remaining missed as_rvalue() in v2.

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