[rs6000] Avoid rotates of floating-point modes

[rs6000] Avoid rotates of floating-point modes

The little-endian VSX code uses rotates to swap the two 64-bit halves of
128-bit scalar modes.  This is fine for TImode and V1TImode, but it
isn't really valid to use RTL rotates on floating-point modes like
KFmode and TFmode, and doing that triggered an assert added by the
SVE series.  This patch uses bit-casts to V1TImode instead.

2017-07-27  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* config/rs6000/rs6000-protos.h (rs6000_emit_le_vsx_permute): Declare.
	* config/rs6000/rs6000.c (rs6000_gen_le_vsx_permute): Replace with...
	(rs6000_emit_le_vsx_permute): ...this.  Take the destination as input.
	Emit instructions rather than returning an expression.  Handle TFmode
	and KFmode by casting to TImode.
	(rs6000_emit_le_vsx_load): Update to use rs6000_emit_le_vsx_permute.
	(rs6000_emit_le_vsx_store): Likewise.
	* config/rs6000/vsx.md (VSX_TI): New iterator.
	(*vsx_le_permute_<mode>): Use it instead of VSX_LE_128.
	(*vsx_le_undo_permute_<mode>): Likewise.
	(*vsx_le_perm_load_<mode>): Use rs6000_emit_le_vsx_permute to
	emit the split sequence.
	(*vsx_le_perm_store_<mode>): Likewise.

From-SVN: r250615