target/m68k: add a mechanism to automatically free TCGv

target/m68k: add a mechanism to automatically free TCGv

SRC_EA() and gen_extend() can return either a temporary
TCGv or a memory allocated one. Mark them when they are
allocated, and free them automatically at end of the
instruction translation.

We want to free locally allocated TCGv to avoid
overflow in sequence like:

  0xc00ae406:  movel %fp@(-132),%fp@(-268)
  0xc00ae40c:  movel %fp@(-128),%fp@(-264)
  0xc00ae412:  movel %fp@(-20),%fp@(-212)
  0xc00ae418:  movel %fp@(-16),%fp@(-208)
  0xc00ae41e:  movel %fp@(-60),%fp@(-220)
  0xc00ae424:  movel %fp@(-56),%fp@(-216)
  0xc00ae42a:  movel %fp@(-124),%fp@(-252)
  0xc00ae430:  movel %fp@(-120),%fp@(-248)
  0xc00ae436:  movel %fp@(-12),%fp@(-260)
  0xc00ae43c:  movel %fp@(-8),%fp@(-256)
  0xc00ae442:  movel %fp@(-52),%fp@(-276)
  0xc00ae448:  movel %fp@(-48),%fp@(-272)
  ...

That can fill a lot of TCGv entries in a sequence,
especially since 15fa08f845 ("tcg: Dynamically allocate TCGOps")
we have no limit to fill the TCGOps cache and we can fill
the entire TCG variables array and overflow it.

Suggested-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180319113544.704-3-laurent@vivier.eu>