i386: Rounding functions TLC

i386: Rounding functions TLC

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

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.