parser: fix match-arm !/? propagation when a match is used as a block value (fix #28000)

[medvednikov]

Fix #28000

A match whose arms use !/? error-propagation, when the match is used as the value of an if-expression (e.g. an if value := opt { … } else { … } option-guard assigned to a variable), emitted invalid C:

error: expected expression
    _t4 = ;

The if-expression's result temp was never assigned the propagated value.

Root cause

Parser.mark_last_call_return_as_used (vlib/v/parser/parser.v) is called for the last statement of an assignment-RHS block and recursively descends into if branches to flag the last call's return value as used (is_return_used = true). It had a case for IfExpr but no case for MatchExpr, so calls in match arms kept is_return_used == false.

In cgen (vlib/v/gen/c/fn.v), the branch that emits the unwrapped value of a propagated call (_t4 = (*(int*)_t5.data);) is guarded by node.is_return_used || is_gen_or_and_assign_rhs. With neither set, cgen wrote only _t4 = followed by nothing.

Fix

Add a MatchExpr case symmetric to the existing IfExpr one, marking the last statement of each match branch.

Reproduction (from the issue)

fn select_value(node ?Node) !int {
	result := if value := node {
		match value {
			First { lower_first(value)! }
			Second { lower_second(value)! }
		}
	} else {
		0
	}
	return result
}

Now compiles and runs correctly.

Tests

• vlib/v/tests/match_as_if_expr_value_with_propagation_test.v — covers ! and ? propagation for a match inside an if-guard, as a direct return value, and assigned to a variable.
• vlib/v3/tests/match_as_if_expr_value_propagation_codegen_test.v — the v3 backend already handled this case correctly; a regression test is added to keep it covered.

Existing vlib/v/compiler_errors_test.v (1617 checker snapshots) and the related match/option/result/or-block test suites pass unchanged.

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[medvednikov]

@codex review

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