Reorganise optimisation level settings, and make the insn shrink pass optional (#1044)

This patch:

* removes the "default" opt level, on the basis that it has no definition and
  is referred to nowhere in the compiler.

* renames the "fastest" level to "none".  The resulting set of transformations
  is unchanged.

* renames the "best" level to "speed_and_size".  The resulting set of
  transformations is unchanged.

* adds a new level, "speed".  This is the same as "speed_and_size" except that
  it omits transformations aimed only at reducing code size.  Currently it
  omits only the insn shrinking pass.

cranelift/filetests/filetests/isa/x86/shrink-multiple-uses.clif

@@ -1,5 +1,5 @@
 test shrink
-set opt_level=best
+set opt_level=speed_and_size
 target x86_64
 
 function %test_multiple_uses(i32 [%rdi]) -> i32 {