Move over patch from Bernd Schmidt from GC branch:

        * emit-rtl.c (gen_rtx_CONST_DOUBLE): New function.
        (gen_rtx): Call it.  Tidy cases.
        * rtl.h (gen_rtx_CONST_DOUBLE): Prototype it.
        * gengenrtl.c: Add commentary.
        (special_rtx): Also match CONST_DOUBLE.
        (gencode): Emit call to memset instead of bzero.

From-SVN: r26110