Use IR node's BranchNode field for IF/ELSE/ENDIF instructions.

Use IR node's BranchNode field for IF/ELSE/ENDIF instructions.

This allows us to back-patch the IF/ELSE instruction's BranchTarget field
to point to the location of the ELSE/ENDIF instructions.  No longer have to
search for ELSE/ENDIF in the interpreter.  Also makes it trivial to translate
IF/ELSE instructions into conditional/unconditional BRA instructions.