i965/fs: Introduce FS IR builder.

i965/fs: Introduce FS IR builder.

The purpose of this change is threefold: First, it improves the
modularity of the compiler back-end by separating the functionality
required to construct an i965 IR program from the rest of the visitor
god-object, what in turn will reduce the coupling between other
components and the visitor allowing a more modular design.  This patch
doesn't yet remove the equivalent functionality from the visitor
classes, as it involves major back-end surgery.

Second, it improves consistency between the scalar and vector
back-ends.  The FS and VEC4 builders can both be used to generate
scalar code with a compatible interface or they can be used to
generate natural vector width code -- 1 or 4 components respectively.

Third, the approach to IR construction is somewhat different to what
the visitor classes currently do.  All parameters affecting code
generation (execution size, half control, point in the program where
new instructions are inserted, etc.) are encapsulated in a stand-alone
object rather than being quasi-global state (yes, anything defined in
one of the visitor classes is effectively global due to the tight
coupling with virtually everything else in the compiler back-end).
This object is lightweight and can be copied, mutated and passed
around, making helper IR-building functions more flexible because they
can now simply take a builder object as argument and will inherit its
IR generation properties in exactly the same way that a discrete
instruction would from the same builder object.

The emit_typed_write() function from my image-load-store branch is an
example that illustrates the usefulness of the latter point: Due to
hardware limitations the function may have to split the untyped
surface message in 8-wide chunks.  That means that the several
functions called to help with the construction of the message payload
are themselves required to set the execution width and half control
correctly on the instructions they emit, and to allocate all registers
with half the default width.  With the previous approach this would
require the used helper functions to be aware of the parameters that
might differ from the default state and explicitly set the instruction
bits accordingly.  With the new approach they would get a modified
builder object as argument that would influence all instructions
emitted by the helper function as if it were the default state.

Another example is the fs_visitor::VARYING_PULL_CONSTANT_LOAD()
method.  It doesn't actually emit any instructions, they are simply
created and inserted into an exec_list which is returned for the
caller to emit at some location of the program.  This sort of two-step
emission becomes unnecessary with the builder interface because the
insertion point is one more of the code generation parameters which
are part of the builder object.  The caller can simply pass
VARYING_PULL_CONSTANT_LOAD() a modified builder object pointing at the
location of the program where the effect of the constant load is
desired.  This two-step emission (which pervades the compiler back-end
and is in most cases redundant) goes away: E.g. ADD() now actually
adds two registers rather than just creating an ADD instruction in
memory, emit(ADD()) is no longer necessary.

v2: Drop scalarizing VEC4 builder.
v3: Take a backend_shader as constructor argument.  Improve handling
    of debug annotations and execution control flags.
v4: Drop Gen6 IF with inline comparison.  Rename "instr" variable.
    Initialize cursor to NULL by default and add method to explicitly
    point the builder at the end of the program.

Reviewed-by: Matt Turner <mattst88@gmail.com>