Optimize empty IR components

[davazp]

Consider this word from test-case.fs:

: test-case ( n -- )
  case
    1 of 10 endof
    2 of 20 endof
    3 of 30 endof
    100 swap
  endcase ;

The IR generated by the compiler (see below) includes some empty components. Components that consist of a single IR_NODE_CONTINUE node.

Those empty components make the code generator generate some jp, to jump between the components. This could be avoided if we implement a coalesce-ir-components optimization to remove those IR components.

========== test-case ( $10AB16F80 ) ========== 

[ $7FCD10602050 ]
LITERAL $1 
CALL over  ( $10AB0C848 )
CALL =  ( $10AB0E9A0 )
0<>IF $7FCD10602160 ELSE $7FCD10602180 

[ $7FCD10602180 ]
LITERAL $2 
CALL over  ( $10AB0C848 )
CALL =  ( $10AB0E9A0 )
0<>IF $7FCD106023F0 ELSE $7FCD10602410 

[ $7FCD10602410 ]
LITERAL $3 
CALL over  ( $10AB0C848 )
CALL =  ( $10AB0E9A0 )
0<>IF $7FCD10602680 ELSE $7FCD106026A0 

[ $7FCD106026A0 ]
LITERAL $64 
CALL swap  ( $10AB0C6E8 )
CALL drop  ( $10AB0C650 )
CONTINUE $7FCD106027B0 

[ $7FCD10602680 ]
CALL drop  ( $10AB0C650 )
LITERAL $1E 
CONTINUE $7FCD106027B0 

[ $7FCD106027B0 ]
CONTINUE $7FCD10602520 

[ $7FCD106023F0 ]
CALL drop  ( $10AB0C650 )
LITERAL $14 
CONTINUE $7FCD10602520 

[ $7FCD10602520 ]
CONTINUE $7FCD10602290 

[ $7FCD10602160 ]
CALL drop  ( $10AB0C650 )
LITERAL $A 
CONTINUE $7FCD10602290 

[ $7FCD10602290 ]
RET