Add sextend_maybe and uextend_maybe to opt ISLE

[scottmcm]

Working on something else, I noticed that the (x < y) != 0 opt was written in a way that only works for C

wasmtime/cranelift/codegen/src/opts/icmp.isle

Lines 16 to 25 in f8c9f67

	;; Optimize icmp-of-icmp.
	(rule (simplify (ne ty
	(uextend _ inner @ (icmp ty _ _ _))
	(iconst_u _ 0)))
	(subsume inner))
	(rule (simplify (eq ty
	(uextend _ (icmp ty cc x y))
	(iconst_u _ 0)))
	(subsume (icmp ty (intcc_complement cc) x y)))

whereas in Rust, which doesn't have the "usual arithmetic conversions", the result of the comparison stays as I8 and thus without the extension it didn't match that pattern.

This picks up a suggestion that alex had made in a previous PR (#7636 (comment)) to make something like lowering ISLE's maybe_uextend extractor for optimization ISLE.

They're then used to improve the mentioned icmp-of-icmp pattern

8ef0b81#diff-fa420f878e321dc6579ac911f9ba8820a9381e4c49615584f6e6909f0e329dfcL124-R125

as well as simplify some of the duplication in the <=> patterns.

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[fitzgen]

Nice!