nir: intel/vec4: Add flag to disable some algebraic optimizations

A couple patches later in this series use the flag to avoid a few
thousand shader-db regresions on all vec4 platforms.

I'm not particularly enamored with the name of this flag.  However, I
suspect the Intel vec4 backend is the only backend that will benefit
from it.  Specifically, the cases where this helps are all cases where
we want to prevent nir_opt_algebraic from rearranging instructions to
create 3-source instructions, such as ffma and flrp, with additional
immediate value or uniform sources.

The earlier commit "intel/vec4: Try to emit a single load for multiple
3-src instruction operands" solves most of the problems caused by
additional immediate values, but the restrictions on register strides
that cause problems for uniforms and shader inputs persist.

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

src/intel/compiler/brw_compiler.c

@@ -82,6 +82,7 @@ static const struct nir_shader_compiler_options vector_nir_options = {
    .lower_unpack_unorm_2x16 = true,
    .lower_extract_byte = true,
    .lower_extract_word = true,
+   .intel_vec4 = true,
    .max_unroll_iterations = 32,
 };