Minor speedup tuning for SecondaryMap (#1020)
Minor speedup tuning for SecondaryMap (#1020)

The `SecondaryMap` abstraction -- basically, resize-on-demand arrays with a
default value -- is very hot in Cranelift.  This small patch is the result of
many profiling runs.  It makes two changes:

* `fn index_mut` is changed to be `#[inline(always)]`, based on profile data.

* `fn index` and `fn index_mut` call `self.elems.resize()` directly, rather
  than via `self.resize()`.  The point of this is not to improve performance.
  Rather, it ensures that the public functions for `SecondaryMap` do not call
  each other.  When public interface functions call each other, it becomes
  difficult to interpret profiling results, because it's harder to see what
  fraction of costs for `SecondaryMap` as a whole come from outside the
  module, and what fraction is the result of "internal" calls to the external
  interface.

The overall result, for wasm_lua_binarytrees, is a 1.4% reduction in
instruction count for the compiler, and a 2.2% reduction in loads/stores.
stoklund/cretonne,stoklund/cretonne,stoklund/cretonne