go/ssa: preserve single-case receive assignment order

[cpunion]

A single-case select is optimized to a simple statement. For a receive
assignment, ordinary assignment lowering evaluates the left-hand side
before the receive. Select semantics require the receive expression and
communication to happen before the assignment left-hand side is
evaluated.

Evaluate the receive before constructing the assignment left-hand side,
while retaining the existing single-case fast path and SSA shape for the
other communication forms. Add interpreter coverage for both one-result
and two-result receive assignments.

Fixes golang/go#80863.

[gopherbot]

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