i965/fs: Define a new fs_builder constructor taking an instruction as argument.

We have a number of optimization passes that repeat the same pattern
before inserting new instructions into the program based on some
previous instruction: They point the default builder at the original
instruction, then call exec_all() and group() to select the same
execution controls the original instruction had, and then maybe call
annotate() to clone the debug annotation from the original
instruction.

In fact an optimization pass missing any of these steps is likely to
be broken if the intention was to emit new code based on a preexisting
instruction, so let's make it easy for passes to do the right thing by
having an fs_builder constructor that automates the task of setting up
a builder to emit a given instruction provided as argument.

The following patches fix all cases I've found in which we weren't
explicitly initializing the execution controls of the emitted
instructions, and clean-up optimization passes which were already
doing the right thing to use the new constructor.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/mesa/drivers/dri/i965/brw_fs_builder.h

@@ -63,6 +63,22 @@ namespace brw {
       {
       }
 
+      /**
+       * Construct an fs_builder that inserts instructions into \p shader
+       * before instruction \p inst in basic block \p block.  The default
+       * execution controls and debug annotation are initialized from the
+       * instruction passed as argument.
+       */
+      fs_builder(backend_shader *shader, bblock_t *block, fs_inst *inst) :
+         shader(shader), block(block), cursor(inst),
+         _dispatch_width(inst->exec_size),
+         _group(inst->force_sechalf ? 8 : 0),
+         force_writemask_all(inst->force_writemask_all)
+      {
+         annotation.str = inst->annotation;
+         annotation.ir = inst->ir;
+      }
+
       /**
        * Construct an fs_builder that inserts instructions before \p cursor in
        * basic block \p block, inheriting other code generation parameters