Use copy_to_reg where appropriate, use int_mode_for_mode and fix
comment indentation.

No functional changes.

2020-12-29  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
    * config/i386/i386-expand.c (ix86_gen_TWO52): Use REAL_MODE_FORMAT
    to determine number of mantissa bits.  Use real_2expN instead
    of real_ldexp.
    (ix86_expand_rint): Use copy_to_reg.
    (ix86_expand_floorceildf_32): Ditto.
    (ix86_expand_truncdf_32): Ditto.
    (ix86_expand_rounddf_32): Ditto.
    (ix86_expand_floorceil): Use copy_to_reg and int_mode_for_mode.
    (ix86_expand_trunc): Ditto.
    (ix86_expand_round): Ditto.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Pushed to mainline.

Uros.