IF / IFxxx       Initiate conditional block                         Directive

  Initiates a conditional block, causing the assembly of statements up to
  the optional ELSE directive, provided that expression is true (evaluates
  to a non-zero value). These directives are available in both Ideal and
  MASM mode.

  IFxxx expression
      truestatements
  [ELSE
      falsestatements]
  ENDIF

  IF expression                 Assemble if expression true
  IF1                           Assemble on pass 1
  IF2                           Assemble on pass 2
  IFB argument                  Assemble if argument is blank (empty)
  IFDEF symbol                  Assemble if symbol is defined
  IFDIF arg1,arg2               Assemble if args different (case-sensitive)
  IFDIFI arg1,arg2              Assemble if args different (case-ignore)
  IFE expression                Assemble if expression is false (0)
  IFIDN arg1,arg2               Assemble if args identical (case-sensitive)
  IFIDNI arg1,arg2              Assemble if args identical (case-ignore)
  IFNB argument                 Assemble if argument is not blank
  IFNDEF symbol                 Assemble if symbol is not defined