7ebff82861
* Optimize sign extension via shifts This commit adds egraph optimization patterns for left-shifting a value and then right-shifting it as a form of sign extending its lower bits. This matches the behavior of the WebAssembly `i32.extend8_s` instruction, for example. Note that the lowering of that WebAssembly instruction does not use shifts, but historical versions of LLVM that didn't support the instruction, or versions with the instruction disabled, will use shifts instead. A second rule for reduction-of-extend being the same as the original value was added to keep an existing shift-related test passing as well. * Add reference assemblies for new opts
Rules here are allowed to rewrite pure expressions arbitrarily,
using the same inputs as the original, or fewer. In other words, we
cannot pull a new eclass id out of thin air and refer to it, other
than a piece of the input or a new node that we construct; but we
can freely rewrite e.g. x+y-y to x.