Also, found/fixed a code generation regression: the emit_swizzle() function
was always returning NULL. This caused emit_move() to miss its chance at peephole
optimization.
IR_LOOP now has two children: the body code, and the tail code.
Tail code is the "i++" part of a for-loop, or the expression at the end
of a "do {} while(expr);" loop.
"continue" translates into: "execute tail code; CONT;"
Also, the test for infinite do/while loops was incorrect.
