v3: lower value-context match/if block operands with !/? propagation (fix #28000)

[medvednikov]

Fix #28000 (v3 backend)

This is the v3-only portion of #28002, split out per request (the vlib/v compiler changes from that PR are dropped here). #28002 is being closed in favor of this.

A match/if used as the value of another expression — a cast, a string-interpolation part, dump(), a prefix/infix operand, an index expression, a selector receiver, or an as operand — must have its (possibly propagating, !/?) branch tails lowered as values. Previously the v3 transformer lowered them in a value-less statement context, which produced an empty ternary / expression for the propagated result.

Change

• vlib/v3/transform/transform.v: add transform_value_operand and is_value_match_or_if_operand helpers. The latter looks through transparent wrappers — (...) parens, unsafe { } blocks, and a trailing expr_stmt (including compositions like i64(unsafe { match ... })). Route the affected operands (string-interp parts, dump(), infix/prefix operands, index children, selector base, cast operands, and block-expr value tails) through target-typed lowering.
• vlib/v3/transform/sum.v: handle a match/if operand of an as cast by materializing it into a value temp first, then re-running the as conversion.

Test

vlib/v3/tests/match_as_if_expr_value_propagation_codegen_test.v — covers !/? propagation for a match used as an if-guard value, and wrapped in parens / unsafe / cast / interpolation / dump / prefix / index / selector operands.

Reproduction from the issue now compiles and runs correctly:

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