checker: minor cleanup of fn_decl

[ttytm]

Merges some redunant checks, checks less taxing conditions first.

Not as relevant for comptime things. Still, tcc compiled it's 1500%+ for e.g.:

b := false
a := 'abcde'
for _ in 0 .. 1_000_000 {
	if a == 'abcde' && b {
		// println('true')
	}
}

vs

- if a == 'abcde' && b {
+ if b && a == 'abcde' {

🤖 Generated by Copilot at f2d5fa6

Refactor and simplify function checking code in vlib/v/checker/fn.v. Move some checks inside a more specific condition and remove unnecessary ones.

🤖 Generated by Copilot at f2d5fa6

• Refactor function checking logic to reduce nesting and simplify conditions (link)

[spytheman]

Good work.