glsl: Add frexp signatures and implementation.

I initially implemented frexp() as an IR opcode with a lowering pass,
but since it returns a value and has an out-parameter, it would break
assumptions our optimization passes make about ir_expressions being pure
(i.e., having no side effects).

For example, if opt_tree_grafting encounters this code:

uniform float u;
void main()
{
  int exp;
  float f = frexp(u, out exp);
  float g = float(exp)/256.0;
  float h = float(exp) + 1.0;
  gl_FragColor = vec4(f, g, h, g + h);
}

it may try to optimize it to this:

uniform float u;
void main()
{
  int exp;
  float g = float(exp)/256.0;
  float h = float(exp) + 1.0;
  gl_FragColor = vec4(frexp(u, out exp), g, h, g + h);
}

Some hardware has an instruction which performs frexp(), but we would
need some other compiler infrastructure to be able to generate it, such
as an intrinsics system that would allow backends to emit specific code
for particular bits of IR.

Reviewed-by: Paul Berry <[email protected]>

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