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.
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