expressions: Fix coercion of numbers to booleans at top level.

expressions: Fix coercion of numbers to booleans at top level.

The check for the expected type in type_check() used the wrong
constant (a node type instead of an expression type) and so the
"if" test was never true.  Also, OP_NUM_TO_BOOLEAN is binary, not
unary, so fixing just the test caused an assertion failure.

This commit fixes both problems and adds a test.

Found by GCC 4.7 report of a type mismatch.