ISLE: add support for implicit =x variable matchers. (#4074)

Currently, a variable can be named in two different ways in an ISLE
pattern. One can write a pattern like `(T x y)` that binds the two
args of `T` with the subpatterns `x` and `y`, each of which match
anything and capture the value as a bound variable. Or, one can write
a pattern like `(T x =x)`, where the first arg pattern `x` captures
the value in `x` and the second arg pattern `=x` matches only the same
value that was already captured.

It turns out (thanks to @fitzgen for this insight here [1]) that this
distinction can actually be inferred easily: if `x` isn't bound, then
mentioning it binds it; otherwise, it matches only the already-bound
variable. There's no concern about ordering (one mention binding
vs. the other) because (i) the value is equal either way, and (ii) the
types at both sites must be the same.

This language tweak seems like it should simplify things nicely! We
can remove the `=x` syntax later if we want, but this PR doesn't do
so.

[1] https://github.com/bytecodealliance/wasmtime/pull/4071#discussion_r859111513

cranelift/isle/isle/isle_examples/fail/bound_var_type_mismatch.isle

@@ -0,0 +1,7 @@
+(type u32 (primitive u32))
+(type u64 (primitive u64))
+
+(decl A (u32 u64) u32)
+
+(rule 1 (A x x) x)
+(rule 0 (A x _) 0)

cranelift/isle/isle/isle_examples/pass/bound_var.isle

@@ -0,0 +1,6 @@
+(type u32 (primitive u32))
+
+(decl A (u32 u32) u32)
+
+(rule 1 (A x x) x)
+(rule 0 (A x _) 0)