Add another e500 subreg pattern.

Add another e500 subreg pattern.

Building glibc for powerpc-linux-gnuspe --enable-e500-double, given
the patch <https://gcc.gnu.org/ml/gcc-patches/2016-11/msg02404.html>
applied, fails with errors such as:

../sysdeps/ieee754/ldbl-128ibm/s_modfl.c: In function '__modfl':
../sysdeps/ieee754/ldbl-128ibm/s_modfl.c:91:1: error: unrecognizable insn:
 }
 ^
(insn 31 30 32 2 (set (reg:DF 203)
        (subreg:DF (reg:TI 202) 8)) "../sysdeps/ieee754/ldbl-128ibm/s_modfl.c":44 -1
     (nil))
../sysdeps/ieee754/ldbl-128ibm/s_modfl.c:91:1: internal compiler error: in extract_insn, at recog.c:2311

This patch adds an insn pattern similar to various patterns already
present to handle extracting such a subreg.  This allows the glibc
build to get further, until it runs into an assembler error for which
I have another patch.

gcc:
	* config/rs6000/spe.md (*frob_<SPE64:mode>_ti_8): New insn
	pattern.

gcc/testsuite:
	* gcc.c-torture/compile/20161123-1.c: New test.

From-SVN: r242813